John Calvin Commentary 2 Corinthians 6:18

"And will be to you a Father, And ye shall be to me sons and daughters, saith the Lord Almighty." — 2 Corinthians 6:18 (ASV)

I will be a Father unto you. This promise does not occur in one passage only, but is repeated in various instances. Paul has added it with this purpose: that recognizing the great honor to which God has exalted us might be a motive to stir us to a more ardent desire for holiness. For when God has restored His Church, which He has gathered from Gentile nations, their redemption is accompanied by this fruit: that believers are seen to be His sons and daughters.

It is no common honor that we are reckoned among the sons of God; it is our responsibility, in turn, to ensure that we do not show ourselves to be degenerate children to Him. For what an injury we do to God if, while we call Him Father, we defile ourselves with the abominations of idols! Therefore, the thought of the high distinction to which He has elevated us ought to sharpen our desire for holiness and purity.