I'm/he's/she's a Christian, so you can trust me/him/her
All the more reason not to trust the person... if the fact that someone is a "born-aginner" even has to be mentioned, then there must be something less-than-honest/honorable that said supposedly-clean-living person has to hide --- "methinks he doth protest too much"!
I have seldom if ever had any more luck with the honesty/integrity/consistency/reliability/kindness of someone about whom I was solemnly assured that, "I'm/he's/she's a Christian, so you can trust me/him/her"... in fact, if anything, these stuck-up Bible-thunpers often seem to be even BIGGER a**h**es than their "non-believer" counterparts! They think that "God will forgive them" for anything they do, and so they make little true effort to keep their word or otherwise treat their fellow humans with even basic decency.