Cornerstone

The cornerstone (or foundation stone) was the key structural element placed at the corner of a building to bind together the two courses of masonry and establish the building's alignment. In ancient construction the cornerstone was carefully selected, set first, and determined the position of the entire structure. The metaphor is used in Scripture both for human leaders — Isaiah 19:13 calls the princes of Egypt its cornerstone — and supremely for the LORD and his anointed servant.

Isaiah 28:16 contains the pivotal text: Behold, I am the one who has laid as a foundation in Zion a stone, a tested stone, a precious cornerstone, of a sure foundation. This is applied to Christ in the New Testament as the stone rejected by the builders but made the cornerstone by God (Psalm 118:22; Matthew 21:42; Acts 4:11; Ephesians 2:20; 1 Peter 2:6–7). Paul describes the church as built on the foundation of apostles and prophets, Christ Jesus himself being the cornerstone (Ephesians 2:20). The image was also used negatively: Isaiah 8:14 and Romans 9:33 describe the Messiah as a stone of stumbling and a rock of offense to those who do not believe.