Sin starts in the heart

“You have heard that it was said, ‘Do not commit adultery.’ But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart.” Matthew 5:27-28
Most of the commands in the Old Testament concerned people’s actions. Do not lie, do not steal, do not commit adultery. This was why it was possible for the rich young man, when Jesus asked him if he had kept the commandment, to reply, “all these I have kept since I was a boy” (Mark 10:20). The Pharisees, the professional law-keepers, thought that they were the most righteous people on earth. After all, they not only kept these laws, but all the laws of the Talmud as well (washing their hands the correct way, not touching Gentiles, and so on). As a result, they became prideful and looked down on the people around them.
But they missed the point. What Jesus is telling us here, is that while the outward action is sinful, this is not where sin begins. He equates looking at a woman lustfully, with adultery. He equates getting angry with another person, with murder (Matt. 5:21-22). We might think these things, but not carry them out. Even so, we still entertained that thought – of wanting to hurt someone, of thinking of how we could tell an untruth, of figuring out how to take something without anybody noticing.
No, sin starts in the heart. We aren’t sinners because we sin. We sin because we are already sinners. We all have a sinful nature inside us, inherited from Adam. Because of this, none of us will be declared righteous apart from Jesus Christ, by our own actions, no matter how many good deeds we do. What we need is for that sinful nature to be replaced. And for that there is only one solution: to believe on Jesus, to invite Him in so that He can give us a new heart.