Nothing is quite so fallacious as to think of sin only in terms of actions; and as long as we think of sin only in terms of things actually done, we fail to understand it. The essence of biblical teaching on sin is that ti is essentially a disposition. It is a state of heart. I suppose we can sum it up by saying that sin is ultimately self-worship and self-adulation; and our Lord shows (what to me is an alarming and terrifying thing) that this tendency on our part to self-adulation is something that follows us even into the very presence of God. It sometimes produces this result; that even when we try to persuade ourselves that we are worshipping God, we are actually worshipping ourselves and doing nothing more.
– Martyn Lloyd-Jones, Studies in the Sermon on the Mount, p. 301.