In the Sermon on the Mount, Jesus makes this last point clear when he names looking after someone with lustful intent as the equivalent to adultery. Here, Jesus warns us against following certain desires. He tells us that desire can become sin. 

We often define ourselves by what we desire most. Be yourself. Fulfill your dream. I can love who I want. But it is not always right to be yourself, nor to fulfill any dream, nor even to love who you want. God cares about how you define yourself, what you dream of, and even whom you sexually desire. 

In the Sermon on the Mount, Jesus makes this last point clear when he names looking after someone with lustful intent as the equivalent to adultery. Here, Jesus warns us against following certain desires. He tells us that desire can become sin. 

While these words may be commonplace to many Christians, we sometimes struggle to grasp the difference between good desire and bad desire as in looking at someone with “lustful intent.” The Holy Spirit has gifted the church with many gifted teachers throughout the ages, and in particular Augustine of Hippo (354–430) was given insight into Jesus’ words here. 

So below, I offer a brief overview of Jesus’ words followed by Augustine’s pastoral reflection to help us understand how our desires can turn into sin so that we can know what is good (good desires) and avoid what is not (sinful desires). 

The Sermon on the Mount

In the first place, it is important to understand the context of Jesus’ words on lustful intent. When the Lord preached his Sermon on the Mount, he fulfilled the law (cf. Matt 5:17). He did so by delivering a new law (as Moses did before) and by pointing to the heart of the law. 

In so doing, Jesus did not internalize the law that was supposedly external under the Old Covenant. The tenth commandment itself forbids even “desiring” something that does not belong to a person. 

Instead, Jesus rightly embraces the already-internalized meaning of the law while also clarifying and giving it an expression that makes sense under the New Covenant. Put simply, the Sermon on the Mount assumes the presence of the Holy Spirit to fulfill its obligations. 

The Old Covenant, while good and holy, excites sin in our bodies as Paul says (Rom 7:8, 12) and contains allowances due to human weakness as Jesus says (Matt 19:8).

The New Covenant does not make any such allowances because it assumes we can fulfill the whole law by the Holy Spirit, that is, by loving God and neighbour as ourselves. 

Hence, Jesus elevates God’s law and imposes a higher standard upon his hearers because he knows that upon receiving the Holy Spirit, we can fulfill the perfect command of the law—the royal law. 

In this context, Jesus explains that lustful desire is sin.

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