Why is it permissible to marry a Christian woman who says, "one of three," while it is impermissible to marry an innovator who commits major shirk? And does one who commits major shirk become an infidel and depart from the fold of Islam if the evidence has not been established against him?

Marrying a chaste woman from the People of the Book is a concession, an exception to the general rule of the prohibition of marrying a non-Muslim woman. There is no doubt about the prohibition of marrying non-Kitābiyyah women (women who are not from the People of the Book). This includes a woman who adheres to an apostatizing innovation (bid‘ah mukaffirah), because an apostate is worse than an original disbeliever. Among the differences between a Kitābiyyah woman and a woman who adheres to an apostatizing innovation is that the former does not originally claim to be Muslim, whereas the latter believes she is Muslim, and this involves ambiguity and confusion. As for someone whose major shirk (definition) (polytheism) takes him out of the fold of Islam without proof being established against him, this applies to matters that are known to be necessities of the religion, but anything else does not result in a ruling of disbelief unless proof is established.