“You have heard that it was said, ‘You shall love your neighbor and hate your enemy.’  But I say to you, Love your enemies and pray for those who persecute you,  so that you may be sons of your Father who is in heaven. For he makes his sun rise on the evil and on the good, and sends rain on the just and on the unjust.  For if you love those who love you, what reward do you have? Do not even the tax collectors do the same?  And if you greet only your brothers, what more are you doing than others? Do not even the Gentiles do the same?  You therefore must be perfect, as your heavenly Father is perfect.
The Meaning of Matthew 5:43-48:
Jesus’ authoritative teaching on love for one’s enemies corrects the disciples’ practice from reciprocation of sinners to imitation of the Father.
Why do I say this is the author’s intended meaning of this passage?
First, “Jesus’ authoritative teaching” reflects “But I say to you” (44). The content of what he says takes up the most space and unifies the passage, so it is the Subject of the passage. That content concerns “love for one’s enemies.” Jesus gives an imperative on loving enemies (44b), with a reasoning related to sonship before the Father (45), and two examples of wrongly reciprocating love and greetings (46-47).
Second, “corrects” reflects the contrast between what the disciples have been taught and believe to be right and what Jesus now teaches. “Corrects” is what Jesus is doing within the entire passage. His “authoritative teaching” is correcting.
Third, “the disciples’ practice from reciprocation of sinner to imitation of the Father” concerns the remainder of the passage. There were some in the listening crowd who were returning love only to those who demonstrated love toward them, and not toward those who did not. Similarly, there were those in the crowd of listeners who greeted only their fellow Jewish brethren and ignored the Gentiles with their greetings. Those are practices of reciprocation: I will give to you only if and what you give to me. Reciprocation concerns justice, i.e., “I will give you what is fair, what is equal, what you are deserving based on your treatment of me or status in life, and no more.” Any “tax collector” and any “Gentile” – both for whom the first century Jewish people had great disdain – could reciprocate, and did so. So any Jewish listener in the crowd was not being righteous by reciprocating, but was acting no better than any thieving tax-collector or any other non-Jew.
However, Jesus intends for citizens of the kingdom of God to be like our heavenly Father—to imitate his works and not the practices of sinners. Unlike the listeners, tax collectors, and Gentiles, the Father does something vastly different than reciprocating. He gives the sun (and all of its benefits) to people who are evil before him. If he gave the sun as reciprocation, no one would get sunlight, heat, or all of the other benefits of the sun! In the same way, the Lord gives rain to people unrighteous in his sight in the same measure that he gives it to people who stand righteous before him. When it comes to sun and rain, the Lord does not give better treatment to his followers than he does to his haters.
What do such actions by the Father show? They show love toward the sinful; his love toward the good and just is assumed.
So then what is “perfect?” To be perfect is to show love – the Father’s love – to those underserving of your love rather than responding to people on the basis of what a just treatment of their behavior or status toward you would deserve. To do so is to be a son of the Heavenly Father (and to do otherwise is to be like a first century Jewish tax collector and Gentile). To be perfect, is to prioritize love over justice in your personal treatment of people.
So this is a passage that calls us to act with mercy and grace toward all. Go pour out sun and rain on those you deem undeserving of such love, even as the Father is doing for each of us this very moment. The cross of Christ and his resurrection from the dead provide the Son and the reign of God for us, in mercy, at the cost of justice poured out on Christ instead of us.
I talk more on the above theory and method of interpretation employed above in Say It! Celebrating Expository Preaching in the African American Tradition (Moody 2020).