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.
Related Commentaries of 1 Corinthians 15:21
1 Corinthians 15:21
21 For since by man came death, by man came also the resurrection of the dead.