Paul draws on a common legal practice to illustrate a spiritual truth. In human terms, once a contract or covenant is legally established, it cannot be altered or annulled by anyone. This analogy is used to emphasize the unchanging nature of God's promises. Specifically, Paul is referring to the covenant God made with Abraham, which was a promise of blessing and salvation through faith. This covenant was not nullified by the Law given to Moses, which came later. Instead, the Law served a different purpose and did not alter the promise of grace given to Abraham.
Paul's message is clear: God's promises are reliable and eternal. They are not subject to change based on human actions or the introduction of new laws. This assurance provides believers with a deep sense of security and confidence in God's faithfulness. It reminds Christians that their faith is grounded in a promise that transcends time and human intervention, reinforcing the idea that salvation and blessing come through faith, not through adherence to the Law.