John Chrysostom

"Be ye therefore perfect, as your Heavenly Father." And He intersperses everywhere abundantly the name of the heavens, by the very place thoroughly elevating their minds. For as yet, I know not how, they were somewhat weak and dull. Seest thou how He pulls up by the roots wrath, and sensual lusts, as well as that of riches, that of glory, all that belongs to this life? For this he had done indeed from the first, but much more now. …

Homily on the Gospel of Matthew 18 ↗

Hilary of Poitiers

Matthew concludes, "All things are perfected by goodness." The law used to demand that your neighbor be loved and allowed hatred against an enemy. Faith, rather, requires that enemies be cherished. It breaks the tendency we have to be peevish and urges us to bear life's difficulties calmly. Faith not only deters anger from turning into revenge but even softens it into love for the injurer. It is merely human to love those who love you, and it is common to cherish those who cherish you. …

Commentary on Matthew 4.27 ↗

Remigius of Rheims

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 ye 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 thee; (Josh. 1:5.) sometimes to express likeness only as here.

Catena Aurea by Aquinas ↗