For Christians: Jesus said that those who sin against the Holy Ghost will not receive forgiveness in this life nor in the next. It seems pretty clear that perhaps there is a limit to forgiveness. But maybe not mercy:
God ejected Cain from his presence by murdering Abel. Cain responded, "Now, people will kill me." If God had no mercy or compassion on Cain he might have said, "Well, you deserve it." But he didn't. Instead, in apparent deference to Cain's fears, he put a mark on him, laying down the law that if anybody killed Cain, vengeance would be taken upon him sevenfold. So, while exacting punishment on Cain, God seemed to show mercy.
I doubt God forgave Cain for Abel's murder, because Cain paid the consequences. It seems mercy and forgiveness are two different things, and this example seems to show that mercy is ever-present.