Here are Commentaries from the ECF’s on Matthew 5:43-48.
You can get them from this website:
Augustine (d 430 AD)–These indeed are examples of the perfect sons of God; yet to this should every believer aim, and seek by prayer to God, and struggles with himself to raise his human spirit to this temper. Yet this so great blessing is not given to all those multitudes which we believe are heard when they pray, Forgive us our debts, as we forgive our debtors.
John Chrysostom (347-407 AD)-- Note through what steps we have now ascended here, and how he has set us on the very pinnacle of virtue. The first step is not to begin to do wrong to any; the second, that in avenging a wrong done to us we be content with retaliating equal; the third, to return nothing of what we have suffered; the fourth, to offer one’s self to the endurance of evil; the fifth, to be ready to suffer even more evil than the oppressor desires to inflict; the sixth, not to hate him of whom we suffer such things; the seventh, to love him; the eighth, to do him good; the ninth, to pray for him. And because the command is great, the reward proposed is also great, namely, to be made like to God, You shall be the sons of your Father which is in heaven.
Jerome (341-420 AD)–**For whoever keeps the commandments of God is thereby made the son of God; he then of whom he here speaks is not by nature his son, but by his own will. **
Augustine–After that rule we must here understand of which John speaks, He gave them power to be made the sons o****f God. One is His Son by nature; **we are made sons by the power which we have received; that is, so far as we fulfill **those things that we are commanded. **So He says not, Do these things because you are sons, but, do these things **that you may become sons. **In calling us to this **then, He calls us to His likeness, for He says, He makes His sun to rise on the righteous and the unrighteous. By the sun we may understand not this visible, but that of which it is said, To you that fear the name of the Lord, the Sun of righteousness shall arise
**Remigius **–Because the utmost perfection of love cannot go beyond the love of enemies, therefore as soon as the Lord has bid us love our enemies, He proceeds, **Be you then perfect, as your Father which is in heaven is perfect. He indeed is perfect, as **being omnipotent - man, as being aided by the Omnipotent. For the word ’ as’ is used in Scripture, sometimes for identity, and equality, as in that, As I was with Moses, so will I be with you
PSEUDO-CHRYSOSTOM–For as our sons after the flesh resemble their fathers in some part of their bodily shape, so do spiritual sons resemble their father God, in holiness.