I’m a simul justus et peccator guy to the end. I know I’ll never be perfect. I cling to Christ and His righteousness. I am weak on sanctification. I do not, however, deny that I quit striving after perfection in the way Saint Paul describes in Philippians chapter three: “forgetting what lies behind and straining forward to what lies ahead, I press on toward the goal for the prize of the upward call of God in Christ Jesus.” Franz Pieper says it best in Volume Three of “Christian Dogmatics”: “But the truth of the imperfection of sanctification in this life is not an excuse for laziness in sanctification and good works. Instead, God’s will and the corresponding Christian attitude to it seeks to ascertain that the Christian strives after not merely a partial, but a complete sanctification and not just some, but all good works.” (page 33 English Translation, page 38 German original. I’ve translated the German original here.)
With this teaching and these examples Christ now concludes [Matthew chapter five]: “You, therefore, must be perfect, as your heavenly Father is perfect.” Here our sophists have spun out many dreams about perfection and have applied them all to their orders and classes—as if only priests and monks were in a state of perfection, the one higher than the other, the bishops higher than all the others, and the pope the highest of all. By this means the word “perfection” becomes completely inapplicable to the ordinary Christian way of life, as if such people could not be called perfect or be perfect. But you hear Christ talking here not to bishops, monks, and nuns, but in general to all Christians who are His pupils, who want to be called the sons of God, and who do not want to be like the publicans and criminals as are the Pharisees and our clergy.
How does it come about that they are perfect? The answer—in brief, because elsewhere I have discussed it in more detail —is this: We cannot be or become perfect in the sense that we do not have any sin, the way they dream about perfection. Here and everywhere in Scripture “to be perfect” means, in the first place, that doctrine be completely correct and perfect, and then, that life move and be regulated according to it. Here, for example, the doctrine is that we should love not only those who do us good, but our enemies, too. Now, whoever teaches this and lives according to this teaching, teaches and lives perfectly.
But the teaching and the life of the Jews were both imperfect and wrong, because they taught that they should love only their friends, and they lived accordingly. Such a love is chopped up and divided, it is only half a love. What He wants is an entire, whole, and undivided love, where one loves and helps his enemy as well as his friend. So I am called a truly perfect man, one who has and holds the doctrine in its entirety. Now, if my life does not measure up to this in every detail—as indeed it cannot, since flesh and blood incessantly hold it back—that does not detract from the perfection. Only we must keep striving for it, and moving and progressing [My note: Luther uses here fortfahre – “continue”. “Progress” would be fortschritte machen] toward it every day. This happens when the spirit is master over the flesh, holding it in cheek, subduing and restraining it, in order not to give it room to act contrary to this teaching. It happens when I let love move along on the true middle course, treating everyone alike and excluding no one. Then I have true Christian perfection, which is not restricted to special offices or stations, but is common to all Christians, and should be. It forms and fashions itself according to the example of the heavenly Father. He does not split or chop up His love and kindness, but by means of the sun and the rain He lets all men on earth enjoy them alike, none excluded, be he pious or wicked.
Luther’s Works Volume 21, pages 128-129