Galatians 3:20

The mention of the word “mediator” implies a contract to which there are at least two parties. But where there is a contract there must be also conditions, and if these conditions are not observed the whole falls to the ground. Such was the Law. The Law was not kept, and therefore the blessings annexed to it were forfeited. On the other hand, the promise depends upon God alone. He gave it, and He will assuredly keep it, no matter what man may do. God alone is concerned in it.
This passage is a conspicuous instance of the advance which has been made in New Testament exegesis. It is said to have received as many as 250 or 300 (according to another estimate, even 430) interpretations, but at the present moment there is a tendency to acquiesce in that given above, which, it is hoped, will be thought satisfactory.

Now a mediator is not a mediator of one. — The very idea of a mediator involves two parties at least. The Law had a mediator, therefore the Law involves two parties. In other words, it is a contract.

But God is one. — On the other hand, God, the giver of the promise, stands alone: therefore the promise is not a contract; and, resting on God, it is indefeasible.