John Calvin Commentary 1 Thessalonians 5:10

"who died for us, that, whether we wake or sleep, we should live together with him." — 1 Thessalonians 5:10 (ASV)

Who died. From the design of Christ’s death, he confirms what he has said. For if he died with this purpose—that he might make us partakers of his life—there is no reason why we should doubt our salvation. It is debatable, however, what he means now by sleeping and waking, for it might seem as if he meant life and death, and this meaning would be more complete. At the same time, we could also suitably interpret it as meaning ordinary sleep.

The sum is this: Christ died with this purpose, that he might bestow his life upon us, a life that is perpetual and has no end. It is not surprising, however, that he affirms that we now live with Christ, since by entering through faith into the kingdom of Christ, we have passed from death into life (John 5:24). Christ himself, into whose body we are ingrafted, makes us alive by his power, and the Spirit that dwells in us is life, because of justification.