John Calvin Commentary 2 Corinthians 6:1

"And working together [with him] we entreat also that ye receive not the grace of God in vain" — 2 Corinthians 6:1 (ASV)

Assisting. He has repeated the instructions of the commission with which God has furnished the ministers of the gospel. After they have faithfully communicated these instructions, they must also make every effort so that these instructions are carried into effect, ensuring their labor is not in vain. They must, I say, add continual exhortations, so that their commission may be efficacious. This is what he means by συνεργοῦντες, (fellow-workers), that is, those devoted to advancing the work; for it is not enough to teach, if you do not also urge.

In this way, the particle σύν would relate to God, or to the commission, which He assigns to His servants. For the doctrine of the gospel is helped by exhortations so that it is not without effect, and ministers connect their efforts with God’s commission, just as it is the role of an ambassador to reinforce with arguments what he presents in his prince's name.

The particle σύν may also be understood as referring to the common efforts of ministers; for if they do the Lord’s work earnestly, they must mutually assist each other. I prefer, however, the former exposition. Chrysostom interprets it as referring to the hearers, with whom ministers are fellow-workers when they rouse them from sloth and indolence.

Ministers are taught here that it is not enough simply to present doctrine. They must also labor for it to be received by the hearers, and not just once, but continually. For as they are messengers between God and men, the first duty incumbent upon them is to offer the grace of God, and the second is to strive with all their might so that it is not offered in vain.