Galatians 3:20

20.Now, a mediator is not a mediator of one. Some are disposed to philosophize on this expression, and would make Paul’s meaning to be, that the twofold nature of Christ is not one in essence. But that Paul is here speaking of the contracting parties, no man of sound judgment entertains a doubt. And so they commonly expound it, that there is no room for a Mediator, unless when one of the parties has a matter to transact with the other. But why that statement should have been introduced they leave undetermined, though the passage manifestly deserves the most careful attention. There may, perhaps, be an Anticipation ( πρόληψις) of some wicked thought that might arise about a change of the divine purpose. Some one might say, “As men, when they change their mind about their covenants, are wont to retract them, so has it happened with the covenants of God.” If you take this to be the meaning, then, in the former clause, Paul would acknowledge that men, who occupy one side of this contract, are unsteady and changeable, while God nevertheless remains the same, is consistent with himself, and partakes not of the unsteadiness of men.

But when I take a closer view of the whole subject, I rather think that it marks a difference between Jews and Gentiles. Christ is not the Mediator of one, because, in respect of outward character, there is a diversity of condition among those with whom, through his mediation, God enters into covenant. But Paul asserts that we have no right to judge in this manner of the covenant of God, as if it contradicted itself, or varied according to the diversities of men. The words are now clear. As Christ formerly reconciled God to the Jews in making a covenant, so now he is the Mediator of the Gentiles. The Jews differ widely from the Gentiles; for circumcision and ceremonies have erected “the middle wall of partition between them.” (Ephesians 2:14.) They were “nigh” to God, (Ephesians 2:13,) while the Gentiles were “afar off;” but still God is consistent with himself. This becomes evident, when Christ brings those who formerly differed among themselves to one God, and makes them unite in one body. God is one, because he always continues to be like himself, and, with unvarying regularity, holds fixed and unalterable the purpose which he has once made. (62)