As Creator of all things, the eternal God exists outside of time. He sees from the very beginning all the way through to the end. He knows every detail of every event. He perceives every thought and intention of the heart. Nothing is outside His awareness.
What a sharp contrast to our limited human perspective! We see what we see–and we guess at what’s going on in a person’s heart. If there’s bad blood with a person, we probably read the worst into her actions and words. If we have a good relationship, we may read the best into what he says and does. The point is, we don’t know people’s hearts. (Perhaps that limited view is why someone commented that we’ll probably be surprised by some of the people we see in heaven–and by some of the folks we don’t.)
Only God knows our human hearts. None of us deserves grace, but our gracious Lord will nevertheless grant mercy and compassion to many. He acts in the world according to His intimate knowledge of each one of us. He is God. . .and we are not!