Christians have debated what the unforgivable sin is for millennia, but we literally have a source which is probably from the apostolic era that describes it in detail:

>And every prophet that speaks in the Spirit you shall neither try nor judge; for every sin shall be forgiven, but this sin shall not be forgiven. But not every one that speaks in the Spirit is a prophet; but only if he hold the ways of the Lord.
Didache chapter 11

So basically this is saying that if you question a genuine prophet while he is speaking in tounges, you CANNOT be forgiven.
Of course there is the possibility that this section of the Didache is later, and hence it wouldn't have the same (or any for that matter) authority, but it's still something to think about.

Slavery
Racism
Sexism
Rape of an unbetrothed woman
Marital "rape"
Sex before marriage (sex is marriage)
Smoking
Polygyny
Early stage abortion