Paul is saying this because he and Timothy were being threatened with death for their sakes. By preaching to the Gentiles they had stirred up hatred from both Jews and Gentiles, risking even death. Commentary on Paul’s Epistles.
So then death worketh in us, but life in you. Your spiritual life, your salvation is produced through faith and grace, but ours by the death of our body. The passion and death of the Apostles has been the life of the Church. "The blood of the martyrs is the seed of the Church," says Tertullian. Chrysostom gives a different explanation: "You live in peace and suffer no such persecutions for the faith as I do; and so you seem to live and I seem to die daily."
We, having the same spirit of faith. As David was hemmed in with dangers, and yet was delivered by God alone from them all, and said. "I believed," i.e, I believe that God will always be true to His promises and deliver me, so too do we believe and hope, and boldly profess that our help and strength, our deliverance and resurrection have been promised by God, and will most surely be wrought out.
Psalm 116, alluded to here by S. Paul, is a Eucharistic Psalm , in which David gives God thanks for his safe deliverance. Hence it begin...
Death worketh in us, when we are under persecutions, and dangers of death, and life in you, who live in ease and plenty. (Witham)
The preaching of the gospel, which we undertake in such a disinterested manner, and which exposes us to so many dangers, is the cause of death to us, but of life to you. It draws down upon us a thousand dangers and disgraces; but procures you all kinds of advantages. You tranquilly enjoy the fruit of our labour, though we do not envy you this happiness, because we hope one day to enjoy the reward of our labours. (Calmet)
Speaking no more of death in the strict sense , but of trials and of rest. 'For we indeed,' he says, 'are in perils and trials, but you in rest; reaping the life which is the fruit of these perils. And we indeed endure the dangerous, but you enjoy the good things; for you undergo not so great trials.'
