Jesus’ answer is the heart (see Matthew 15:19). From the heart come all kinds of evil thoughts and desires. That is where adultery breeds. That is where plans are made to win the heart of someone who belongs to another. That is where the desire for illicit comfort and pleasure springs forth.
And such desires are sinful and wicked. They are a form of unfaithfulness. An objectification of another person, treating a fellow human being as if they exist solely for my needs. Whether heterosexual or homosexual, any desire for sexual relations with someone who is not your spouse is contrary to God’s will.
It is important to emphasize that we are focusing on sexual desires. It is not necessarily lust when we recognize that someone is beautiful or if we are attracted to someone/enjoy being in their company. 1 Love between a man and a woman is celebrated in Scripture (see Song of Solomon 1:1–8:14). It is a good thing for a man to find a wife (Proverbs 18:22), we are glad when friendships become engagements, and engagements lead into marriage. But if we undress someone in our minds or view them as an object for our fantasies, that is definitely overstepping the line.
On this topic, we must be careful of blaming women for the lust of men. That is something that has been typical throughout history. In ancient times, women were expected to wear head coverings in order to prevent lust and make it clear they were already married. Though the Jews recognized that lust was inappropriate, some cultures considered it to be a perfectly healthy and normal practice, and head coverings were a means of bringing some control. While it is true that what we see has an influence on what we think and desire, the fundamental problem lies within and not without. Human bodies are not inherently sinful. We must learn to look at others as fellow human beings and not objects for our pleasure. Even if people dress in a way that is deliberately provocative/meant to entice, there is never an excuse for lust.
And so, while we wait for Jesus’ return, while we seek to live as kingdom citizens in this world, here is the call of our Saviour: instead of indulging our sinful desires, we must make it our aim to fight against them. If your right eye causes you to stumble, gouge it out and throw it away. Be ruthless in your fight against sin.
Reading about eyes and hands, we should be careful of thinking that Jesus is calling for amputation. There are some Christians who have done so (for example, Origen), but that is to misunderstand what he says. If evil arises in the heart, it cannot be solved with amputation. Jesus’ words are thus hyperbolic commands. He is emphasizing the decisive actions that are needed to fight against sinful desires. They are strong, they are powerful, they must not be tolerated, much less indulged.
On this topic, notice also the assumption of Jesus. He assumes that kingdom citizens will struggle with lust while they live in this world. He assumes that sinful desires will continue to spring forth from our heart even after regeneration/conversion. There is no promise of perfection in the here and now, and that means all citizens must make wise and appropriate plans to combat lust. For example, moving my computer to a public spot in the house so that everyone can see what I am looking at on the screen. Reminding myself of God’s truth before I go to the beach so that I do not undress the people I see. Waiting for marriage before moving in with my partner. Checking the content of films or series before I watch them. Staying away from novels which celebrate sin and encourage sexual fantasy. There are lots of things that can be done to avoid temptation and we are wise to do so. It is part of what it means exercise self-control and pursue the fruit of the Spirit (see Galatians 5:22–24).
27 “You have heard that it was said, ‘You shall not commit adultery.’