1 Thessalonians 3:10

"For what thanksgiving can we render again unto God for you, for all the joy wherewith we joy for your sakes before our God; night and day praying exceedingly that we may see your face, and may perfect that which is lacking in your faith?"

Not only, he says, are ye the causes of life to us, but also of much joy, and so much that we cannot worthily give thanks to God. Your good behavior, he says, we consider to be the gift of God. Such kindnesses have you shown to us, that we think it to be of God; yea, rather, and it is of God. For such a disposition of mind comes not of a human soul or carefulness.

"Night and day," he says, "praying exceedingly." This too is a sign of joy. For as any husbandman, hearing concerning his land that has been tilled by himself, that it is burdened with ears of grain, longs with his own eyes to see so pleasant a sight, so Paul to see Macedonia. "Praying exceedingly." Observe the excess; "that we may see your face, and may perfect that which is lacking in your faith."

Here there is a great question. For if now thou livest, because they stand fast, and Timothy brought thee "glad tidings of their faith and love," and thou art full of so much joy as not to be able worthily to give thanks to God, how sayest thou here that there are deficiencies in their faith? Were those then the words of flattery? By no means, far be it. For previously he testified that they endured many conflicts, and were no worse affected than the Churches in Judaea. What then is it? They had not enjoyed the full benefit of his teaching, nor learned all that it behoved them to learn. And this he shows toward the end. Perhaps there had been questionings among them concerning the Resurrection, and there were many who troubled them, not by temptations, nor by dangers, but by acting the part of teachers. This is what he says is lacking in their faith, and for this reason, he has so explained himself, and has not said, that you should be confirmed, where indeed he feared concerning the faith itself, "I have sent," he says, "Timothy to confirm you," but here, "to perfect that which is lacking," which is rather a matter of teaching than of confirming.

But every discourse on this topic, where one’s goal is that what is said may not only be believed but also understood and known, is burdensome for those still spiritually immature. These the apostle says are carnal, needing to be nourished with milk, as they do not have the strength to perceive such things and are more easily frustrated than fed. Because of this it happens that spiritual men do not in all circumstances refuse to discuss these things with the carnal, on account of the Catholic faith, which must be preached to all. Yet, having the desire to transmit some degree of truth to understandings not capable of a secure grasp, they take care to transmit these in a way that does not cause their discourse to be regarded with disdain and the truth contained therein to be completely ignored. Thus, in order to continue to nourish them despite their immaturity, [the apostle], writing to the Colossians, says, “Even if I am absent in body, in spirit I am with you, rejoicing and beholding your order and that which is lacking to your faith in Christ.” And to the Thessalonians, “Night and day,” he says, “more abundantly praying that we may see your face and may supply the things that are lacking to your faith.”

Let the admonition of the most blessed apostle not depart from your hearts, “If anyone preaches to you a gospel besides that which you have received, let him be anathema.” He did not say “more than you received” but “besides that which you have received.” For if he said the former, he himself would be prejudging himself, who desired to come to the Thessalonians that he might supply what things were lacking to their faith. But he who supplies adds to what was smaller, he does not take away what was there. But he who goes beyond the rule of faith does not go forward in the way but goes back to the way.

Note the intensification in the expression! "Praying most earnestly" to see them, just as a farmer, hearing that the field he cultivates abounds with fruit, impatiently desires to see it with his own eyes.

If above he testified to their "standing," then how does he now say: "what was lacking in your faith"? They did not benefit from all the teaching, and they were lacking, perhaps, knowledge about the resurrection and other similar matters. Also, there were many false teachers, which is why he said: "what was lacking." Where he feared for the faith itself, he says: "we sent Timothy to establish you" (v. 2); but here he says: "to supply," which pertains more to teaching than to establishing. For we call imperfect that which lacks only a little.

Then when Paul says, night and day, the third effect of their relationship is explained. First, he points out the frequency of his prayer; secondly, he shows what he desires while praying (3:11). So Paul insists: We give thanks for things past; nevertheless we do not fail to pray also for future concerns, indeed, we do so night and day, that is, in adversity and prosperity. "Evening and morning and at noon I utter my complaint and moan" (Ps. 55:17). To supply what is lacking in your faith: not matters that pertain to the fundamentals of the faith, but some special teachings which the Apostle did not preach to them at their birth: "I, brethren, could not address you as spiritual men, but as men of the flesh" (1 Cor. 3:1). "I have yet many things to say to you, but you cannot bear them now" (Jn. 16:12).