May 5. On this date in 1943, a letter written in behalf of Shoghi Effendi said "The friends should by all means be encouraged to withdraw from church membership and be made to realize that, though we as Bahá'ís are ardent believers in Christ, we do not and cannot support, church institutions and doctrines when Christ has come again and brought new laws for the world today and its present needs; to adhere to forms, mostly man-made, and now out-moded and no longer needed, is meaningless. This does not mean they should no longer associate with the church members; they should cease to be registered members of it."

May 5. On this date in 1943, a letter written in behalf of Shoghi Effendi said "The friends should by all means be encouraged to withdraw from church membership and be made to realize that, though we as Bahá'ís are ardent believers in Christ, we do not and cannot support, church institutions and doctrines when Christ has come again and brought new laws for the world today and its present needs; to adhere to forms, mostly man-made, and now out-moded and no longer needed, is meaningless. This does not mean they should no longer associate with the church members; they should cease to be registered members of it."

533. Bahá'ís Are Ardent Believers in Christ
"The friends should by all means be encouraged to withdraw from church membership and be made to realize that, though we as Bahá'ís are ardent believers in Christ, we do not and cannot support, church institutions and doctrines when Christ has come again and brought new laws for the world today and its present needs; to adhere to forms, mostly man-made, and now out-moded and no longer needed, is meaningless. This does not mean they should no longer associate with the church members; they should cease to be registered members of it."
(From a letter written on behalf of Shoghi Effendi to an individual believer, May 5, 1943)

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!’

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)