For now we live. Here it appears still more clearly that Paul almost forgot himself for the sake of the Thessalonians, or, at least, making regard for himself a mere secondary consideration, devoted his first and chief thoughts to them. At the same time he did not do that so much from affection to men as from a desire for the Lord's glory. For zeal for God and Christ glowed in his holy breast to such a degree that it in a manner swallowed up all other anxieties. "We live," says he, that is, "we are in good health, if you persevere in the Lord." And under the adverb now, he repeats what he had formerly stated, that he had been greatly pressed down by affliction and necessity; yet he declares that whatever evil he endures in his own person does not hinder his joy. "Though in myself I am dead, yet in your welfare I live." By this all pastors are admonished what sort of connection ought to subsist between them and the Church -- that they reckon themselves happy when it goes well with the Church, although they should be in other respects encompassed with many miseries, and, on the other hand, that they pine away with grief and sorrow if they see the building which they have constructed in a state of decay, although matters otherwise should be joyful and prosperous.

For now we live, if ye stand fast in the Lord - This is equivalent to saying, "My life and comfort depend on your stability in the faith, and your correct Christian walk;" compare Martial 6:70. Non est vivere, sed valere, vita - "Life consists not merely in living, but in the enjoyment of health." See also Seneca, Epis. 99, and Manilius, 1-Thessalonians 4:5, as quoted by Wetstein. The meaning here is, that Paul now enjoyed life; he had that which constituted real life, in the fact that they acted as became Christians, and so as to show that his labor among them had not been in vain. The same thing here affirmed is true of all faithful ministers of the gospel. They feel that they have something that may be called life, and that is worth living for, when those to whom they preach maintain a close walk with God.

For now we live - Your steadfastness in the faith gives me new life and comfort; I now feel that I live to some purpose, as my labor in the Lord is not in vain.

For now we (a) live, if ye stand fast in the Lord.
(a) For now you cannot otherwise think of me as at rest and in a good state of being, unless you go forward in religion and faith.

For now we live,.... Before they were dead men, lifeless, disconsolate, dispirited, carrying about with them the dying of the Lord Jesus, and death working in them, and they, as it were, under the sentence of that, being killed all the day long for Christ's sake; but now, upon this news, in the midst of all their sore trials and troubles, their spirits revived, and they became alive and cheerful; see Psalm 22:26, it was like life from the dead unto them:
if ye stand fast in the Lord: or "our Lord", as the Syriac and Ethiopic versions read; that is, "in the faith of the Lord", as the Arabic version renders it: they were in the Lord secretly by electing grace, and openly by regenerating grace, and they abode in him; and by persevering grace, they were rooted and built up in Christ, and established in the faith of him, of his person, office, and grace; they were steady in the exercise of grace upon him, and stood fast in the liberty wherewith he had made them free, and continued steadfastly in the doctrines and ordinances of the Gospel; for the "if" here is not expressive of doubting, but of reasoning, "seeing ye stand fast in the Lord"; of which they were assured by Timothy: and this gave them fresh spirit and life amidst the deaths in which they often were.

now--as the case is; seeing ye stand fast.
we live--we flourish. It revives us in our affliction to hear of your steadfastness (Psalm 22:26).
if--implying that the vivid joy which the missionaries "now" feel, will continue if the Thessalonians continue steadfast. They still needed exhortation, 1-Thessalonians 3:10; therefore he subjoins the conditional clause, "if ye," &c. (Philippians 4:1).

Now we live - Indeed; we enjoy life: so great is our affection for you.