King James Bible
"Now a mediator is not a mediator of one, but God is one."
Paul argues that mediation by definition requires at least two parties—a mediator stands between distinct groups to facilitate agreement. This refers back to Moses mediating the law between God and Israel at Sinai, emphasizing the bilateral nature of that covenant. In contrast to the two-party arrangement of the law, God's promise to Abraham was unilateral—depending solely on God's faithfulness, not human performance. Paul invokes the Shema's declaration of God's oneness to show that the promise operates differently from the mediated law.