....who is to say that a 'Christian' monk wasn't 'donated' by his family but didn't believe in the New Religion?

well - most novices who were pledged by their families entered very young (a sizeable percentage even as infants) and "indoctination" would have been very hard to avoid ewven for older youths.

The Jews in Spain and other regions who went "undercover" found this out, because they lost a high percentage of their youth to the masquarade - even though they were doing their best to educate the youngsters in the faith to which they were born.

Novices to Christian monastaries would also have lacked opputunities to even learn about other faiths.

Remember - religion permeated every facet of life during those times. there was no seperation of church and anything, let alone church and state.