For the love of Christ constraineth us; because we thus judge, that if one died for all, then were all dead: For the love of Christ constraineth us - We have the love of God shed abroad in our hearts, and this causes us to love God intensely, and to love and labor for the salvation of men. And it is the effect produced by this love which συνεχει ἡμας, bears us away with itself, which causes us to love after the similitude of that love by which we are influenced; and as God so loved the world as to give his Son for it, and as Christ so loved the world as to pour out his life for it, so we, influenced by the very same love, desire to spend and be spent for the glory of God, and the salvation of immortal souls. By the fear of God the apostles endeavored to persuade and convince men, and the love of Christ constrained them so to act.
If one died for all, then were all dead - The first position the apostle takes for granted; viz. that Jesus Christ died for All mankind. This no apostolic man nor primitive Christian ever did doubt or could doubt.
The second position he infers from the first, and justly too; for if all had not been guilty, and consigned to eternal death because of their sins there could have been no need of his death. Therefore, as he most certainly died for All, then all were dead, and needed his sacrifice, and the quickening power of his Spirit.