1 John 3:16-24 (ESV)
By this we know love, that he laid down his life for us, and we ought to lay down our lives for the brothers.  But if anyone has the world's goods and sees his brother in need, yet closes his heart against him, how does God's love abide in him?  Little children, let us not love in word or talk but in deed and in truth.
 By this we shall know that we are of the truth and reassure our heart before him;  for whenever our heart condemns us, God is greater than our heart, and he knows everything.  Beloved, if our heart does not condemn us, we have confidence before God;  and whatever we ask we receive from him, because we keep his commandments and do what pleases him.  And this is his commandment, that we believe in the name of his Son Jesus Christ and love one another, just as he has commanded us.  Whoever keeps his commandments abides in him, and he in them. And by this we know that he abides in us, by the Spirit whom he has given us.
“For whenever our heart condemns us, God is greater than our heart, and he knows everything.” Our hearts, cesspools of sin, and the chief sin of this cesspool is unbelief. Yes unbelief is sin, it is the chief of sins from which all other sin flows. Our hearts will condemn us every chance they get. They will mislead us to place our faith in anything but Christ and his death and resurrection. They will lead us to trust in warm fuzzies, burning bosoms, the fact that we donated five dollars to the salvation army at Christmas from all the goods of the world we have, or whatever other work we care to hang our faith and assurance of salvation or election on. That is the work of our heart. Sin. Unbelief. Condemnation. Our heart uses every trick it can to lead us into false belief (which is at its root unbelief) despair and other great shame and vice. But God is greater than our heart. He overcomes our heart. He cleans our heart and gives us a good conscience, not because of what we do, want to do, or have done, but because of Christ who forgives our sins, and gives us His Spirit which leads us to believe in him and love one another. Loving one another we point each other to Christ and his cross in which our salvation is as certain as the fact that he rose on the third day.