Is there one sin you can't be forgiven?

Of course not! The God you are referring to does not hold grudges, get mad, or do anything of the sort. Remember when Jesus was dying on the cross, and the criminal next to him repented and stated his belief, and Jesus forgave him instantly. God loves you, no matter how terrible the crime you have committed against him is. Rape, murder, abuse, etc. Even GENOCIDE. If you are truly sorry, you will be forgiven. But you can't be sorry for the sole purpose of wanting to go to heaven. You have to be REALLY sorry.

Roman Catholic AnswerUnfortunately, yes, there are sins which can not be forgiven: the sins "against the Holy Ghost". They are typically enumerated: Presumption of God's mercy, Despair, impugning the known truth, obstinacy in sin, and final impenitence. The common thread in all of these sins is a failure to be humble, a failure to admit that you are sinner, and a failure to genuinely ask God - in all humility - for forgiveness; a failure to recognize that you need forgiveness. This state is all too common in our modern society. These are all "ultimate sins" so to speak which can not be forgiven. God will not save us against our will. And our will can not be demanding, in presumption, that God better save us!

Furthermore to ask in humility is to ask with the right intention, that is the intention to stop sinning and to live according to God's law. To ask for forgiveness and then go right on doing what you have been doing is presumption of the worst kind - and is unforgivable.