Galatians 3:20

20 A mediator, however, implies more than one party; but God is one.

Read Galatians 3:20 Using Other Translations

Now a mediator is not a mediator of one, but God is one.
Now an intermediary implies more than one, but God is one.
Now a mediator is helpful if more than one party must reach an agreement. But God, who is one, did not use a mediator when he gave his promise to Abraham.