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)

 'Abdu'l-Bahá outwardly lived his life as a Muslim, attending Friday prayers, and on his death was given a Muslim burial.

In 1905, in response to a Commission of Inquiry, 'Abdu'l-Bahá wrote the sultan a letter protesting that his followers refrain from involvement in partisan politics and that his tariqa had guided many Americans to Islam.
 

In 1921, 'Abdu'l-Bahá had encouraged the Bahá’ís of Manchester to emulate the dissimulation practiced by 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.