This is the stone which was set at nought of you builders, which is become the head of the corner.
“The stone” is Jesus Christ, and this is a reference to Psalm 118:22, where the builders are the rulers of Israel. Peter says “you builders”, placing the men of the Sanhedrin clearly in that role. Peter is saying, “By your rejection and crucifixion of Jesus Christ, you have fulfilled one of your own prophecies. And, just as surely as one prophecy has been fulfilled, so shall the other – the rejected stone will become the head of the corner!
Jesus Christ the Rock
Christ is the Rock of Salvation: Exo. 17:1-7
Christ is the Rock of Judgment: Isa. 8:14; 1 Pet. 2:8
Christ is the Rock of Provision: Isa. 26:3,4
Christ is the Foundation Rock: Isa. 28:16; Psalm 118:22
Christ is the Rock of the Church: Matt. 16:16; 1 Cor. 3:11; Eph. 2:20-22.
Christ is the Destroying Rock at the 2nd Advent: Dan. 2:35, He’s the Rock cut out without hands that crushes the image.
The Jews understood these references; and Peter is telling them that they have “set at nought” the Lord, having rejected Him and totally disregarded Him.
Christ “became” the “head of the corner,” referring to His joining the two spiritual kingdoms, the Church and born-again Israel. After the 2nd Advent, there will also be political unity, but of course made up only of believers in Christ.