Just for the record, Merriam Webster defines "faith":
2 a (1) : belief and trust in and loyalty to God (2) : belief in the traditional doctrines of a religion b (1) : firm belief in something for which there is no proof (2) : complete trust 3: something that is believed especially with strong conviction; especially: a system of religious beliefs
Can't speak for Islam, but Christians often use "faith" as a synonym for religion; i.e., "marrying within one's own faith".
All that being said, Donald asks a good question.
Not much difference between Islam and Christianity in this matter.