Jesus said in the Sermon on the Mount, “Be perfect, therefore, as your heavenly Father is perfect” (Mt 5:48).
This is often interpreted as a call to sinless perfection, something that Christians cannot attain prior to glorification. As such it’s used as a hermeneutical key to understanding the Sermon on the Mount as a whole. In this view, Jesus is not laying out the way of life for his followers. Instead, he is setting the bar so far out of their reach that they must turn to him for mercy and find acceptance in his righteousness.
I fully embrace the theological conclusions of this position: Christians cannot live sinlessly in this life and can only be accepted by God on the basis of the imputed righteousness of Jesus. However, I don’t think this text teaches that.
There are three reasons for understanding this verse as something that every Christian should and can obey.
First, the word perfect (τέλειος) often has a different meaning than we commonly have in mind when we talk about perfection in theological contexts. It means mature or complete (e.g., 1 Cor 2:6, Phil 3:15, Heb 5:14). It doesn’t necessarily mean without sin.
Second, the context makes it clear what Jesus meant by this call to perfection.
You have heard that it was said, “Love your neighbor and hate your enemy.” But I tell you, love your enemies and pray for those who persecute you, that you may be children of your Father in heaven. He causes his sun to rise on the evil and the good, and sends rain on the righteous and the unrighteous. If you love those who love you, what reward will you get? Are not even the tax collectors doing that? And if you greet only your own people, what are you doing more than others? Do not even pagans do that? Be perfect, therefore, as your heavenly Father is perfect.
Jesus is discussing our responsibility to love not only our neighbors, but also our enemies and those who do us harm. The reason is that God is kind to all his creatures. Loving only your neighbors or those who love us back is incomplete love. Instead, our love should be complete, just like the Father’s love is complete. Jesus’ main point here is that we are to treat others with the same kind or quality of love as God does, not that we quantitatively do so to the same degree of sinless perfection.
Finally, the parallel passage in Luke 6:26 reads, “Be merciful, just as your Father is merciful.” Here’s the full context:
If you love those who love you, what credit is that to you? Even sinners love those who love them. And if you do good to those who are good to you, what credit is that to you? Even sinners do that. And if you lend to those from whom you expect repayment, what credit is that to you? Even sinners lend to sinners, expecting to be repaid in full. But love your enemies, do good to them, and lend to them without expecting to get anything back. Then your reward will be great, and you will be children of the Most High, because he is kind to the ungrateful and wicked. Be merciful, just as your Father is merciful.
This parallel passage makes the same point as Matthew’s Gospel. The concluding statement sheds light on the meaning of Matthew 5:48. What Matthew is getting at by taking about being perfect is equivalent to what Luke means when he talks of being merciful. To be like our Father, we need to be merciful and love with a mature, complete kind of love that doesn’t restrict itself to those who love us back.
Herman Ridderbos articulates this point well in When the Time Had Fully Come: Studies in New Testament Theology, 30–31 (Jordan Station, ON: Paideia, 1982):
[A]s for Matthew 5:48, Jesus does not, in any universal sense, demand of man moral equality with God. The word “perfect” as used here denotes quite a different meaning. It concerns the “perfectness,” the consistency of love. Man is bound not only to love his neighbor but also his enemies. It is in this sense that the heavenly Father, too, is perfect. “For he maketh his sun to rise on the evil and the good, and sendeth rain on the just and the unjust” (Matt. 5:45). There is no room in His love for half measures. Hence perfect love is also demanded from His children, not partial, not only towards friends, but enemies as well. Hence also Luke can add in the corresponding passage in his Gospel: “Be ye merciful, even as your Father is merciful” (Luke 6:36). “Even as” means “equally perfect,” “equally consistent.” Therefore, it is not possible to appeal to this to contend the positive tenor of the law in the Sermon on the Mount. It belongs to the essential quality, I might well say to the logic of the Kingdom of the Heavens, that a disciple of Jesus does not content himself with love merely towards his fellows. There is no question of straining the moral demands ad absurdum.
In The Coming of the Kingdom, ed. Raymond O. Zorn, trans. H. de Jongste, 246n86 (Philadelphia, PA: Presbyterian and Reformed, 1962), he makes the same point more concisely:
[I]n [Matthew] 5:48 teleioi must be taken in a formal sense, viz., as perfect, consistent, not giving up when only half finished. For the subject here is love which must not be restricted to those alone who are of the same mind. This is why in Luke 6:36 this meaning can be rendered by the words, “Be ye therefore merciful, as your Father is also merciful.”