“Be perfect, therefore, as your heavenly Father is perfect.” (NIV). This is another challenging statement which Jesus makes in the “sermon on the mount”. However I think there are a few things which shed light on this statement. Often, words in other languages have different shades of meaning which can’t be exactly translated. As I understand, the Greek word used here carries the idea of maturity, being complete, finished, and fully grown more so than flawlessness. I believe we could also think of it as being fully, correctly human.
Once again, I believe the context also sheds light on this passage. At first glance verse 48 can appear to be an isolated axiom, unrelated to the surrounding content. But I don’t believe this is actually the case. In verses 43-47, Jesus addresses how people love some but hate others. I think what Jesus is getting at here is that this type of behavior demonstrates inconsistency of character. He gives the example that the Father loves everyone regardless their standing. In this way the Father is consistent and perfect.
I think this understanding of being perfect applies to other points Jesus addresses in chapter 5. It is inconsistent to condemn murder but to harbor anger against someone, or to condemn adultery but lust after someone. If we are consistent, complete, and mature, we don’t need to try persuade others by making oaths. Rather our “yes” will be yes and our “no” no. Paul communicates this same idea in Romans 2. This also fits with the rest of scripture (For example, see James 3:9-12 specifically, and actually most of the rest of James too).
In a large way, the idea of maturity sums up the overall idea of what Jesus is teaching from verse 21 of chapter 5 through the end of the sermon.