These are hard lessons to flesh and blood. But if we are thoroughly grounded in the faith of Christ's love, this will make his commands easy to us. Every one that comes to him for washing in his blood, and knows the greatness of the mercy and the love there is in him, can say, in truth and sincerity, Lord, what wilt thou have me to do? Let us then aim to be merciful, even according to the mercy of our heavenly Father to us.
Jesus himself, on his cross, when he prayed that his murderers might be forgiven, for they knew not what they were doing, and his true servant Stephen, who copied faithfully his Lord in his own dying moments, are beautiful though extreme examples of what is meant here. It is St. Luke alone who mentions this act of Jesus on the cross; it is St. Luke, again, who has preserved St. Stephen's words, uttered while they were stoning him to death. He would show how the Lord's command could be carried out.
[Excerpt from Matthew Henry's Concise Commentary & The Pulpit Commentary]