Do Christians believe that they can be forgiven for anything?

I ask this out of curiosity, not to be snarky. Assuming that a person believes in Christ, are there limits to what he can be forgiven for? John F Kennedy and Martin Luther King engaged in adultery. If they apologized and asked God for forgiveness, is it reasonable to suppose that they were granted entry into Heaven? What if you kill someone, but then turn around and act morally? If, at the end of is life, Hitler saw the error of his ways and asked for forgiveness, would it have been granted? In short, does the only thing that matters is your final attitude, even if you are so incapacitated that you are no longer capable of doing anything evil?