Do you agree that you shouldn't try to refute someone's beliefs without fully understanding them first?

Tells you you're not saved because you're Calvinist? I think you were talking to someone on fire for Christ and still on milk rather than meat. If he was misrepresenting TULIP, then maybe it's best to pull back and just celebrate what you hold in common. Those on milk are still your brothers & sisters in the Lord, and they need to be taught slowly, not all at once at a bus stop. They may have a lifetime of unlearning to do in their Arminian/Pelagian thinking.