Christ tells his disciples, Love your enemies and pray for those who persecute you, so that you may be children of your Father in heaven . . . Be perfect, therefore, as your heavenly Father is perfect (Mt 5.44, 48). What does 'perfect' mean?
It is well-known that the word 'perfect' in the ancient languages (i.e., Latin and Greek) carries the sense of completion. In Greek the word used here is τέλειοι, which is clearly related to the noun τέλος. A thing's τέλος is its goal, its end, the thing or range of activity to which it aims naturally. Consequently for a thing to be τέλειος is for it to have accomplished its purpose, to be fulfilled, to be fully realized. Imagine a machine that is accomplishing its purpose completely and flawlessly, or a tree operating naturally and efficiently to produce healthy and plentiful fruit -- that is what is being suggested here.
Likewise in Latin, the word perfectus, from which we get our English 'perfect', is composed of two elements: the preposition per-, which means 'through', and -fectus, which means made. A thing that is perfectus in Latin is that has been made or realized "all the way through," to the end. Imagine constructing something after a model, and going step by step until the product is entirely finished with nothing leftover. That is something perfectus, made through to the end, complete. A sketch or drawing is one thing, but a painting with all the finishing touches made is perfectus.
Jesus teaches us, therefore, that when we love our enemies in addition to our friends, we are perfect -- we are complete, we are realized all the way through. Our love, too, is complete when we do these things. The implication is that it is a part of God's model for humanity that a human person love each and all, irrespective of merit; this is how Jesus teaches us a person is to be considered and evaluated. Humanity's goal is to love all, and a human is perfect and reaches this goal when he does this. The goal, of course, is no less than God himself, who loves all indifferently. Loving in this unconditional and universal way is consequently a part of being the image and likeness of God!
Now Christ as the image of the invisible God (Col 1.15) certainly fulfills this function himself in his life. As Stăniloae and other theologians emphasize, Christ as the perfected man represents the goal of all humanity; he is the one who, in Athanasius' terms, remakes the image and likeness of God in his own human nature and transforms us in this direction as well. His perfected love is shown through the fact that he has loved enemy and friend alike in his act of atonement: he dies for us while we were still sinners (Rom 5.8), and in fact he is the atonement for the sins of the whole world (1 John 2.2).
We, too, become true humans when we love in this way. That is what Jesus teaches us. We become true humans, which means we become like God, when we lay down our lives for one another (cf. 1 John 3.16).