Perhaps now, more than ever, Christians need wisdom to process the multitude of temptations to sexual sin with which they are confronted. While it is true that sexual sin has always been a problem in the church, there should be little doubt that the lust of the flesh, the lust of the eyes and the pride of life are a seemingly ubiquitous danger for Christians today.
The Puritans were well-known for their diagnosis of sin. In fact, it might be one of their lasting legacies. Some modern theologians (e.g. see this and this) have continued that pattern of examining the Christian life by seeking to uncover the root issues which lie behind our external sins. Of course, Scripture itself is the main source for uncovering both surface and root issues. Below are several biblical principles by which we may guard ourselves from sexual sin.
1. Sexual sin is idolatry: The Apostle Paul tells us this plainly in Colossians 3:5: “Put to death therefore what is earthly in you: sexual immorality, impurity, passion, evil desire and covetousness, which is idolatry.” That is to say, sexual immorality–whether on a screen or in person–is a replacement god. Nothing should be more appalling and grevious for the sincere Christian, than to turn his or her back on Christ and bow down to another god. That is precisely what we do, however, in idolatry. We de-throne Almighty God and replace him with pornography; or fantasy; or adultery.
2. Sexual sin occurs when we fail to “keep our heart.” Proverbs 4:23-27 provides us with a powerful warning and encouragement to help us keep our hearts pure. “Keep the heart with all vigilance, for from it flows the springs of life” (Prov 4:23). The next verses tell us what that looks like: v 24 watch what you and others say; v 25 watch what you look at; v 26 watch what you think about and vs 27 watch where you go. That is to say, if we are not always keeping guard over our senses, we allow ourselves to become subject to wickedness. We strangle the ministry of the Spirit in our lives (c.f. Prov 4:23 & John 7:37), giving ourselves to impurity, through which the Spirit will never work.