Perhaps I should have said "former" colony, but I see your point. Accepting such a petition for membership could easily lead to miss understandings.A non-member world's colony trying to join the UFP would, I think, be rejected out of prudence.
I could see a wealthy organization like the Catholic Church sponsoring a colony, or assisting in the finance as a partner. As a person of faith it seems to me that a colony group that wasn't sponsored by a chuch would, once estabished, request the presents of say a mission or a church primate to help organize their community. Having at least one Priest (Rabbi, Imam, Guru) in the groups "first wave" might be a standard part of a colonial charter. If a group was looking to escape from religion this person would be omitted.
I could find this where?"The Catholic Church in the Age of the Federation"