John Calvin Commentary 2 Corinthians 8:5

"and [this], not as we had hoped, but first they gave their own selves to the Lord, and to us through the will of God." — 2 Corinthians 8:5 (ASV)

And not as He expected from them an ordinary degree of willingness, such as any Christian should show; but they went beyond his expectation, since they not only had their material resources in readiness, but were prepared to devote even themselves. They gave themselves, he says, first to God, then to us.

It may be asked whether their giving themselves to God and to Paul were two different things. It is quite a common occurrence that when God charges or commands through anyone, He associates the person whom He employs as His minister, both in the authority to command and in the obedience that is given.

It seemed good to the Holy Spirit and to us (Acts 15:28); say the Apostles, while at the same time they merely, as instruments, declared what had been revealed and commanded by the Spirit. Again, The people believed the Lord and his servant Moses (Exodus 14:31), while at the same time Moses had nothing apart from God. This, too, is what is meant by the clause that follows—by the will of God. For, as they were obedient to God, and had committed themselves to Paul's ministry to be regulated by his counsel, they were influenced by this consideration in listening to Paul, as speaking from God’s mouth.