December 23

An important distinction, however is drawn between association with other movements and actual membership. In general Bahá'ís are encouraged to collaborate with all others who are working towards the same goals as the Faith. Bahá'ís are not permitted, however, to be members of certain secret societies, of the religious organizations of other Faiths, of political organizations or, of course, of organizations whose goals are in conflict with the Bahá'í principles. For example, Bahá'ís would gladly work together with Christians in humanitarian activities, but a Bahá'í, believing in Bahá'u'lláh, cannot be a member of a Christian church which believes that Christ has not yet returned. 
(From a letter dated February 14, 1993 written on behalf of the Universal House of Justice to an individual believer)