But is not the Son God? Most truly He is; why then does he say, “One God”? In contradistinction to the idols; not to the Son. For he is discoursing about truth and error.
Now a mediator ought to have communion with both parties, between whom he is to mediate. For this is the property of a mediator, to be in close communion with each of those whose mediator he is. For he would be no longer a mediator, if he were connected with one but separated from the other. If therefore He partakes not of the nature of the Father, He is not a Mediator, but is separated. For as He is partaker of the nature of men, because He came to men, so is He partaker of the nature of God, because He came from God.
Because He was to mediate between two natures, He must approximate to the two natures; for as the place situated between two others is joined to each place, so must that between natures be joined to either nature. As therefore He became Man, so was He also God. A man could not have become a mediator, because he must also plead with God. God could not have been mediator, since those could not receive Him, toward whom He should have mediated. And as elsewhere he says, “There is one God the Father, … and one Lord Jesus Christ” (1 Cor. 8:6); so also here “One” God, and “One” Mediator; he does not say two; for he would not have that number wrested to Polytheism, of which he was speaking. So he wrote “One” and “One.” You see how accurate are the expressions of Scripture! For though one and one are two, we are not to say this, though reason suggests it. And here thou sayest not one and one are two, and yet thou sayest what reason does not suggest. …“For there is one God,” he says, “and one Mediator between God and men, the Man Christ Jesus.”
Covenant Presbyterian Church (OPC)
Hammond, WI, 54015