Matthew 5:43-48
43 Ye have heard that it hath been said, Thou shalt love thy neighbour, and hate thine enemy.
44 But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;
45 That ye may be the children of your Father which is in heaven: for he maketh his sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust.
46 For if ye love them which love you, what reward have ye? do not even the publicans the same?
47 And if ye salute your brethren only, what do ye more than others? do not even the publicans so?
48 Be ye therefore perfect, even as your Father which is in heaven is perfect.
The Grace New Testament Commentary - Matthew 5:44-48
5:44-48 Followers of Christ are to refrain from hating their enemies, but they must also do the opposite: love them, bless them, "do good to" them, and "pray for" them. The reasons for such unnatural behavior are twofold.
First, Christ's disciples behave this way toward their enemies to demonstrate to whom they belong. Unrestricted love identifies believers as "sons of your Father in heaven." They are related to God who displays love toward His enemies. This does not mean that those who love their enemies become children of God because of their behavior. That would mean works are necessary for justification (see Rom 4:5). These verses parallel the truth of Matt 5:9 that godly behavior results in others recognizing the believer as a son of God.
Second, this behavior demonstrates the distinctive level of commitment that befits sons of God. If believers love only their friends, are they any better than the godless pagans of society? With regard to love the ideal is to be "perfect, just as (their) Father in heaven is perfect."
The word perfect (teleios) sometimes means "mature" (e.g., 1 Cor 14:20; Eph 4:13; Heb 5:14). But that cannot be the case here, for such a translation removes the force of Christ's statement. Furthermore, how is God the Father mature, whole, and complete in a way that people can be like Him? It is better to take the word perfect in its normal sense of sinless righteousness. This is what God is (1 John 1:5), and what believers should be as well. Such a shocking statement is a proper conclusion to the section of the sermon on pharisaical tradition. The section begins with Christ's statement in Matt 5:20 that perfect righteousness is God's requirement for entrance into the kingdom of heaven. The implied question was, "How can anyone be that righteous?" Jesus gives six examples on how perfection is revealed, and then concludes by saying that if believers can do these things they will be perfect.
Those who try to attain such perfection by their works should now be sufficiently deflated. No one can attain such righteousness on his or her own. The only way any person can become as perfect as the Father in heaven is if God gives His righteousness to him. For those who have already believed in Jesus for eternal life, however, Jesus does not lower the standard; He presents it as an ideal to aim for even though He knows people will still fall short. And even then, in context, the goal is not external righteousness, but the inner purity and love Christ has been extolling (cf. 1 Pet 1:13-16).