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

[HeavenlyOne]

I think that passage is speaking of unrepentant folks, not just people who have done so in the past or struggle with it. When we repent, God not only forgives, He forgets. When we ask forgiveness again, God doesn't remember that we just asked forgiveness for the same thing yesterday, the day before, 1000 times last year, etc. It's as if we are asking for the first time.

Since we don't know the hearts of others, it's not for us to judge, but I think we know when someone is repentant and when they aren't. I believe it's the latter that Paul is addressing, and I agree with him.