“Then those also who have died
ἄρα καὶ οἱ κοιμηθέντες ἐν Χριστῷ ἀπώλοντο.
Paul said then those also who have died in Christ (ἄρα καὶ οἱ κοιμηθέντες ἐν Χριστῷ) have perished (ἀπώλοντο). Stating the reverse, Paul said that if there was no resurrection, anyone who died in Christ would have perished without any eternal life. If they gave up their life for Christ, it would have been in vain because there was no eternal life. They simply fell asleep, perished and died. Paul indicated that there was no eternal life is there is no resurrection. Do you know anyone who does not believe in an afterlife?