How do we know the eternal covenant is unchangeable?

The unchangeability of the eternal covenant is grounded in the character of God. As Hebrews 6:18-19 states, it is impossible for God to lie, providing a strong foundation for the assurance that His promises will endure. This is further illustrated in the story of Esther, where decisions made by kings could not be altered, showcasing a lesser authority compared to God's immutable covenant. The everlasting covenant, unlike the edicts of human kings, is rooted in the sovereign will of God, who decreed it in the councils of eternity. Thus, believers can rest in the assurance that their salvation and security are anchored in God's unchanging promises.