Justified Subjective Belief

I met this guy recently, Eric, who prefers to make the distinction between being a Christian and a Paulian. As a Christian, he doesn't try and defend Paul's writings (or scripture in general), but stands by Christ's central teachings. It's questionable what Christ's central teachings are (and how original they are), but Eric says it's just about love. He sees this as objectively defensible, but it seems like the teaching is dependent on the character of Christ: if Jesus was just a person, believing in "Love" is not being "Christian", it's also being Buddhist, Confucianist, and a host of other things. On the other hand, if Jesus is God incarnate, He has a privileged understanding of the nature of humanity and the ethics that follow. Arguing for the latter requires a shareable (objectively defensible) argument for Christ's God-nature, which would have to be founded in scripture.

A more general question that arises from this regards the nature of personal belief. Eric believes more than he can share (again, objectively defend), but isn't worried about not being able to share it. This concerns me: when, if ever, are we justified in believing something about the nature of reality we can't defend to someone else?