The verse speaks to the nature of God's covenant with humanity, contrasting it with human agreements that require a mediator. A mediator is necessary when there are two distinct parties, often with differing interests or needs. However, God is one, highlighting His singular nature and the unique way He relates to us. This reflects the idea that God's promises and covenants are not dependent on human mediation but are direct and personal. This oneness of God assures us that His promises are steadfast and reliable, not subject to the complexities of human negotiation.
In the broader context of the letter to the Galatians, Paul is addressing the role of the law and faith. He emphasizes that while the law was given through intermediaries, God's promise to Abraham—and by extension to all believers—is fulfilled directly through faith in Christ. This underscores the accessibility of God's grace and the personal relationship each believer can have with Him. It is a reminder of the simplicity and purity of faith, free from the need for additional mediators, and highlights the unity and consistency of God's purpose and love for humanity.