Since the heart is only known by God, how can we be SURE?

[stmatthew]

The forgiveness of God was extended at the death of Christ. It is not appropriated by us until we "bow the knee".

It is kind of like me forgiving a person that has did me wrong without their asking for it. I am free from their deed, but until they ask me they are not. So likewise God has already extended forgiveness, but we are not free until we ask for it.