What does Mark 12:10 mean?

Mark 12:10–11 is a direct quote of the Septuagint version of Psalm 118:22–23. Jewish tradition states that the "stone" was rejected for use in building the temple sanctuary but became essential for the porch. It's unclear if the stone is for the cornerstone of the foundation or the keystone of the arch. Rabbis taught that the stone represented either Abraham, David, or the Messiah, and often interpreted "the builders" to mean the experts of the Mosaic law.

The sanctuary of the temple is the interior portion behind the altar. This includes steps, a vestibule, the nave, and the Holy of Holies. The porch is the main entrance to the temple on the east end. Ezekiel saw a vision in which the glory of the Lord will pass through the East Gate and enter the temple through the porch (Ezekiel 43:1–5). That the stone was rejected from being a part of the sanctuary means the Jewish religious leaders reject Jesus as a significant part of their religion and relationship with God. That same stone becomes a part of the porch, through which all God-followers will be welcomed. God is not stymied by our misinterpretations of His will, whether intentional or by mistake. He will fulfill His plan and when we look back, we will call it "marvelous" (Mark 12:11).