John Calvin Commentary Acts 2:18

"Yea and on my servants and on my handmaidens in those days Will I pour forth of my Spirit; and they shall prophesy." — Acts 2:18 (ASV)

Upon my servants. In these words the promise is restricted to the worshippers of God. For God does not profane His Spirit, which He would do if He were to make it common to the unbelieving and despisers.

It is certain that we are made the servants of God by the Spirit; and therefore, we are not His servants until we have received it. But first, those whom God has adopted into His family, and whom He has formed by His Spirit to obey Him, He furnishes with new gifts afterward.

Again, the prophet was not focused on that order, but his meaning was to make this grace proper to the Church alone. Since the Church was then only among the Jews, He honorably calls them the servants and handmaids of God.

But after God gathered to Himself a Church from all peoples, the wall of separation being pulled down, all who are received into the society of the covenant are called by the same name. Only let us remember that the Spirit is appointed specifically for the Church.