"You, O Lord... know the hearts of all" Acts1:24
These words can be either scary or heart-warming. If there is evil in my heart -- that is, the intention to know, speak or do evil, then I think I would be a little worried to know that God knows what's in my heart. But if my intentions are to do what's right but it doesn't always happen that way, then I would be comforted. I'm so glad that God knows how weak we are and how sin dogs our footsteps, because I believe He knows and understands our failures and always stands ready to forgive our weaknesses -- and even our deliberate sins, that's what forgiveness is all about.
But there is a warning, God says, "my spirit will not always strive with men". So if we deliberately sin, thinking all we have to do is ask for forgiveness afterwards, we must beware that God will not be mocked. His forgiveness is not given out cheaply. Repentance must come from the heart, even though we know we will most likely sin again, so does He. We are all sinners. He knows that, but he also knows what is in our hearts, whether we have a desire to please Him or not. The desire to please God is what is important here.