John Calvin Commentary 1 Thessalonians 3:12

"and the Lord make you to increase and abound in love one toward another, and toward all men, even as we also [do] toward you;" — 1 Thessalonians 3:12 (ASV)

And the Lord fill you. Here we have another prayer—that meanwhile, while his way is obstructed, the Lord, during his absence, may confirm the Thessalonians in holiness and fill them with love. And from this, we again learn what constitutes the perfection of the Christian life—love and pure holiness of heart, flowing from faith. He recommends love mutually cherished towards each other, and afterwards towards all, for as it is fitting that we should begin with those who are of the household of faith (Galatians 6:10), so our love ought to extend to the whole human race. Furthermore, while closer relationships must be cherished, we must not overlook those who are more distant from us, lest they be prevented from holding their proper place.

He desires the Thessalonians to abound in love and be filled with it, because as we make progress in our knowledge of God, the love for our fellow believers must also increase in us, until it takes possession of our whole heart, with the corrupt love of self being rooted out.

He prays that the love of the Thessalonians may be perfected by God, intimating that its increase, just like its beginning, was from God alone. Therefore, it is evident how preposterously those act who measure our strength by the precepts of the Divine law. The end of the law is love, says Paul (1 Timothy 1:5), yet he himself declares that it is a work of God.

Therefore, when God marks out our life, He does not consider what we can do, but requires from us what is beyond our strength, so that we may learn to ask Him for the power to accomplish it. When he says—as we also towards you, he stimulates them by his own example.