May 21. On this date in 1933, a letter from Shoghi Effendi to an individual believer stated that "No Bahá'í can any longer dissimulate his faith and practise the laws and ordinances of a previous dispensation and call himself at the same time a believer. No compromise, no vacillation can any longer be tolerated. We must have the courage of our convictions and preserve the integrity of our glorious Cause."

 

May 21. On this date in 1933, a letter from Shoghi Effendi to an individual believer stated that "No Bahá'í can any longer dissimulate his faith and practise the laws and ordinances of a previous dispensation and call himself at the same time a believer. No compromise, no vacillation can any longer be tolerated. We must have the courage of our convictions and preserve the integrity of our glorious Cause."

531. We Must Have the Courage of Our Convictions

"...No Bahá'í can any longer dissimulate his faith and practise the laws and ordinances of a previous dispensation and call himself at the same time a believer. No compromise, no vacillation can any longer be tolerated. We must have the courage of our convictions and preserve the integrity of our glorious Cause...."

(Postscript by the Guardian to a letter written on his behalf to an individual believer, May 21, 1933)

During his visit to London in 1911, 'Abdu’l-Bahá had the following interaction with a Christian...

A student of the modern methods of the higher criticism asked ‘Abdu’l-Bahá if he would do well to continue in the church with which he had been associated all his life, and whose language was full of meaning to him. ‘Abdu’l-Bahá answered: “You must not dissociate yourself from it. Know this; the Kingdom of God is not in any Society; some seekers go through many Societies as a traveller goes through many cities till he reach his destination. If you belong to a Society already do not forsake your brothers. You can be a Bahá’í-Christian, a Bahá’í-Freemason, a Bahá’í-Jew, a Bahá’í-Muḥammadán. The number nine contains eight, and seven, and all the other numbers, and does not deny any of them. Do not distress or deny anyone by saying ‘He is not a Bahá’í!’

During his tour of North America in 1912, coverage in news stories would report this comment.

For example, on February 18, 1912, an article in "The Pittsburgh Pennsylvania Press" featured an article stating "In some respects the Bahá’í movement is the most remarkable of modern times. It isn’t a religion, in the sense that Christianity and Mohammedism and other faiths are religions. Its followers belong to many diverse sects, remaining Christian or Mohammedan or Brahmin as the case may be, and still being thorough going Bahis." During his visit to London in 1911, 'Abdu’l-Bahá had stated that "You can be a Bahá’í-Christian, a Bahá’í-Freemason, a Bahá’í-Jew, a Bahá’í-Muḥammadán."

And on February 28, 1912, the "SFO Daily News" of San Francisco featured an article stating "In some respects the Bahá’í movement is the most remarkable of modern times. It isn’t a religion in the sense that Christianity and Mohammedism and other faiths are religious. Its followers belong to many diverse sects, remaining Christian or Mohammedan or Brahmin as the case mya be, and still being thorough going Bahá’ís" During his visit to London in 1911, 'Abdu’l-Bahá had stated that "You can be a Bahá’í-Christian, a Bahá’í-Freemason, a Bahá’í-Jew, a Bahá’í-Muḥammadán."

Similarly, on September 1, 1912, "The Oregonian" of Portland, Oregon carried an article about ‘Abdu’l-Bahá, introducing him as the "Leader of Religions Movement Which Claims Three Million Followers" and quoting his statement "When in London he was approached by a student of higher criticism who asked ‘Abdu’l-Bahá if he should continue in the church. ‘Abdu’l-Bahá replied: “Yes, you must not dissociate yourself from it. Know this: the Kingdom of God is not in any society. If you belong to a society already do not forsake your brothers. You can be a Bahá’í-Christian, a Bahá’í-Freemason, a Bahá’í-Jew, Bahá’í-Mohammedan.”


In fact, 'Abdu'l-Bahá had encouraged the Bahá’ís of Manchester to emulate the freemasons...

October 1921
In the Name of God! O Sincere Servant of the Blessed Beauty! 

Your detailed letter has been received, and has been the cause of the utmost joy and gladness. Praise be to God! The loved ones are all astir and active, but prudence is necessary. You have rent the veil too widely asunder. Explain to the loved ones that the rending of the veil to such an extent will be the cause of great agitation, and the harm thereof will reach to the Holy Land. Great caution is necessary. Discourses in churches and great public gatherings are in no wise permitted as in this place enemies, within and without, are lying in wait and are bent on aggression. Prudence requires that activity should, for the present, be concealed and carried on with the utmost moderation. Convey to the loved ones, one and all, on my behalf, the greatest longing, love and kindness. Give a spiritual message from me to Mr. Healds and say unto him: “Peruse the Gospel, how His Holiness Christ – may my life be a sacrifice to Him – says, ‘Conceal it, that the Pharisees may not be informed thereof.’ Now the same condition prevails.”

This matter is of the greatest importance. On no account let them contribute articles to the newspapers, and so long as they are not sure of any soul, let them breath no word to him. Consider how the Freemasons have for two hundred years carried on their work, and unto this day they have not openly declared it to any soul. Not until they find a hearing ear will they speak. The loved ones too must proceed with the greatest prudence, lest serious difficulties be created. If any one should travel to the Holy Land, he must on no account declare to anyone by the way that his purpose is to visit us. The loved ones must, in the presence of strangers, speak forth simply the teachings of the Blessed Beauty and mention no word of the belief concerning Him. Should anyone inquire, “What is your belief regarding the Blessed Beauty?” let them answer: “We regard Him as the foremost teacher and educator of these later ages and Abdu’l-Baha as the Centre of His Covenant.”

The original scanned document can be found at http://www.h-net.org/~bahai/docs/vol13/Barstow_600-623.pdf , where it is labeled BC#608. It was published through H-Net's Documents on the Shaykhi, Babi and Baha'i Movements

Numerous statement by Shoghi Effendi would later contradict these statements, with Bahá’ís being explicity prohibited in associating with any other faiths.
For example...

1387. Bahá'ís Belonging to Churches, Synagogues, Freemasonry and the Like
"As regards the question of Bahá'ís belonging to churches, synagogues, Freemasonry, etc., the friends must realize that now that the Faith is over 100 years old, and its own institutions arising, so to speak, rapidly above-ground, the distinctions are becoming ever sharper, and the necessity for them to support whole-heartedly their own institutions and cut themselves off entirely from those of the past, is now clearer than ever before. The eyes of the people of the world are beginning to be focused on us; and, as humanity's plight goes from bad to worse, we will be watched ever more intently by non-Bahá'ís, to see whether we do uphold our own institutions wholeheartedly; whether we are the people of the new creation or not; whether we live up to our beliefs, principles and laws in deed as well as word. We cannot be too careful. We cannot be too exemplary.
"There is another aspect to this question which the friends should seriously ponder, and that is that, whereas organizations such as Freemasonry may have been in the past entirely free from any political taint, in the state of flux the world is in at present, and the extraordinary way in which things become corrupted and tainted by political thought and influences, there is no guarantee that such an association might not gradually or suddenly become a political instrument. The less Bahá'ís have to do, therefore, with such things, the better."
(From a letter written on behalf of Shoghi Effendi to the National Spiritual Assembly of the British Isles, August 5, 1955)

And also...
1389. Why Bahá'ís Are Requested to Withdraw from Membership in the Church, Synagogue, etc.

"The point is not that there is something intrinsically wrong with Masonry, which no doubt has many very high ideals and principles, and has had a very good influence in the past.
"The reasons why the Guardian feels that it is imperative for the Bahá'ís to be dissociated from masonry at this time, and I might add, other secret associations, is that we are the building blocks of Bahá'u'lláh's New World Order ... the Bahá'ís should be absolutely independent, and stand identified only with their own teachings. That is why they are requested to withdraw from membership in the church, the synagogue, or whatever other previous religious organization they may have been affiliated with, to have nothing whatsoever to do with secret societies, or with political movements, etc. It protects the Cause, it reinforces the Cause, and it asserts before all the world its independent character.
"Another reason is that unfortunately the tremendous political influences in the world today are seeping deeper and deeper into men's minds; and movements which in the past were absolutely uninfluenced by any political tinge of thought now in many places are becoming infiltrated with political side-taking and political issues; and it becomes all the more important for the Bahá'ís to withdraw from them in order to protect the Faith.
"The Guardian believes that you, as an intelligent man, a Bahá'í, will see the need for this. It is only by all living according to general principles that we can knit the fabric of the Faith all over the world into a closer unity.
"He is fully aware that certain individuals are struck much more forcibly by such requests than others. This has been the case with some of the old Bahá'ís in England, who have been Masons from their boyhood on; but, as it is his duty to protect the Faith, he can only appeal to the Bahá'ís to assist him in doing so; and to consider the general good, rather than their personal feelings, however deep they may be, in such matters."
(From a letter written on behalf of Shoghi Effendi to an individual believer, February 12, 1956)

Numerous statements by Shoghi Effendi and the Universal House of Justice would later contradict these statements, with Bahá’ís being explicity prohibited in associating with any other faiths.

On July 24, 1954, a letter written on behalf of Shoghi Effendi to the Spiritual Assembly of Japan stated that "So far as non-Bahá'ís affiliating with the Bahá'í Faith is concerned, either a person becomes a Bahá'í and accepts Bahá'u'lláh as the divine Manifestation for this day, or he does not...they can only become Bahá'ís on the basis of accepting Bahá'u'lláh as a divine Manifestation, and of course, with this goes the acceptance of the Bab as the Forerunner, and Abdu'l-Bahá as the Center of the Covenant, and the present Administrative Order. When a person has reached the sea of immortality, it is idle to keep seeking elsewhere."

1384. Affiliation with Faith Alone is Insufficient
"So far as non-Bahá'ís affiliating with the Bahá'í Faith is concerned, either a person becomes a Bahá'í and accepts Bahá'u'lláh as the divine Manifestation for this day, or he does not. The tenets of the Bahá'í Faith are simple as outlined by the Guardian, but they do not permit of any variations. In other words, if any members of the ... Movement wish to become Bahá'ís, they will be most welcome; but they can only become Bahá'ís on the basis of accepting Bahá'u'lláh as a divine Manifestation, and of course, with this goes the acceptance of the Bab as the Forerunner, and Abdu'l-Bahá as the Center of the Covenant, and the present Administrative Order.
"When a person has reached the sea of immortality, it is idle to keep seeking elsewhere...."
(From a letter written on behalf of Shoghi Effendi to the Spiritual Assembly of Japan, July 24, 1953: Japan Will Turn Ablaze, pp. 76-77)

For example, on August 5, 1955, Shoghi Effendi addressed a letter to an individual believer...

1387. Bahá'ís Belonging to Churches, Synagogues,Freemasonry and the Like
"As regards the question of Bahá'ís belonging to churches, synagogues, Freemasonry, etc., the friends must realize that now that the Faith is over 100 years old, and its own institutions arising, so to speak, rapidly above-ground, the distinctions are becoming ever sharper, and the necessity for them to support whole-heartedly their own institutions and cut themselves off entirely from those of the past, is now clearer than ever before. The eyes of the people of the world are beginning to be focused on us; and, as humanity's plight goes from bad to worse, we will be watched ever more intently by non-Bahá'ís, to see whether we do uphold our own institutions wholeheartedly; whether we are the people of the new creation or not; whether we live up to our beliefs, principles and laws in deed as well as word. We cannot be too careful. We cannot be too exemplary.
"There is another aspect to this question which the friends should seriously ponder, and that is that, whereas organizations such as Freemasonry may have been in the past entirely free from any political taint, in the state of flux the world is in at present, and the extraordinary way in which things become corrupted and tainted by political thought and influences, there is no guarantee that such an association might not gradually or suddenly become a political instrument. The less Bahá'ís have to do, therefore, with such things, the better."

And also...

1389. Why Bahá'ís Are Requested to Withdraw from Membership in the Church, Synagogue, etc.
"The point is not that there is something intrinsically wrong with Masonry, which no doubt has many very high ideals and principles, and has had a very good influence in the past.
"The reasons why the Guardian feels that it is imperative for the Bahá'ís to be dissociated from masonry at this time, and I might add, other secret associations, is that we are the building blocks of Bahá'u'lláh's New World Order ... the Bahá'ís should be absolutely independent, and stand identified only with their own teachings. That is why they are requested to withdraw from membership in the church, the synagogue, or whatever other previous religious organization they may have been affiliated with, to have nothing whatsoever to do with secret societies, or with political movements, etc. It protects the Cause, it reinforces the Cause, and it asserts before all the world its independent character.
"Another reason is that unfortunately the tremendous political influences in the world today are seeping deeper and deeper into men's minds; and movements which in the past were absolutely uninfluenced by any political tinge of thought now in many places are becoming infiltrated with political side-taking and political issues; and it becomes all the more important for the Bahá'ís to withdraw from them in order to protect the Faith.
"The Guardian believes that you, as an intelligent man, a Bahá'í, will see the need for this. It is only by all living according to general principles that we can knit the fabric of the Faith all over the world into a closer unity.
"He is fully aware that certain individuals are struck much more forcibly by such requests than others. This has been the case with some of the old Bahá'ís in England, who have been Masons from their boyhood on; but, as it is his duty to protect the Faith, he can only appeal to the Bahá'ís to assist him in doing so; and to consider the general good, rather than their personal feelings, however deep they may be, in such matters."
(From a letter written on behalf of Shoghi Effendi to an individual believer, February 12, 1956)

On November 21, 1968, a letter of the Universal House of Justice to the National Spiritual Assembly of Switzerland stated that "If a person is registered as a member of a church or similar religious organization he should withdraw from it on becoming a Bahá'í...one cannot be a Bahá'í and also a member of another religious organization...For a Bahá'í to be a member of a community which holds such beliefs is disloyalty to Christ and hypocrisy towards the Christians."

532. On Becoming a Bahá'í Should Withdraw from Church
"If a person is registered as a member of a church or similar religious organization he should withdraw from it on becoming a Bahá'í.
"In the case of new believers, it should be made clear to them in the course of teaching them the Faith that one cannot be a Bahá'í and also a member of another religious organization. This is simply a matter of straight-forwardness and honesty. A great part of the teaching of Jesus Christ concerned His Second Coming and the preparation of His followers to be ready for it. The Bahá'ís believe He has come. No Christian Church believes this; on the contrary, they either look for Him still, or have ceased to believe that He will come. For a Bahá'í to be a member of a community which holds such beliefs is disloyalty to Christ and hypocrisy towards the Christians.
"You should not formalize the method by which the withdrawal from the church is to be made, and certainly nothing should be added to a declaration form, if you use one. It should be left to the Local Spiritual Assembly which is accepting the declaration to satisfy itself, as it deems best in each case, that the new believer has already resigned from the church, or does so within a reasonable time of his declaration.
"In regard to the old believers, your Assembly should tactfully, and in a kindly way, make the Bahá'í position clear to them and gently persuade them to resign from their former churches. This is a matter for great tact and discretion. If such a believer remains adamant you will have to consider depriving him of his voting rights."
(From a letter of the Universal House of Justice to the National Spiritual Assembly of Switzerland, November 21, 1968: Canadian Bahá'í News, Special Section, March 1973, p. 6)