This sermon centers on the unwavering faithfulness of God in fulfilling His eternal covenant with Abraham and Sarah, emphasizing that divine promises are certain, confirmed, and rooted in grace rather than human effort. Through the transformation of Sarah's name and the miraculous promise of Isaac's birth despite their advanced age, the text highlights God's sovereignty, His certainty in keeping His word, and the distinction between salvation by faith—exemplified in Isaac, the child of promise—and salvation by works, represented by Ishmael. The preacher underscores that God's care extends even to those outside the covenant, yet His everlasting covenant is exclusively established with those who share Abraham's faith, affirming that true inheritance comes not through human striving but through faith in God's redemptive plan fulfilled in Christ. The application calls believers to rest in God's perfect timing, find comfort in His confirmed promises, and persevere in faith, knowing that the same faith that sustained Abraham now sustains the Church as an eternal, unbreakable covenant.