Charles Ellicott Commentary 1 Corinthians 15:21

"For since by man [came] death, by man [came] also the resurrection of the dead." — 1 Corinthians 15:21 (ASV)

For since by man . . .—The image of the firstfruits is followed up by an explanation of the unity of Christ and Humanity. The firstfruit must be a sample of the same kind as that which it represents. That condition is fulfilled in the case of the firstfruits of the resurrection.