In his second rhetorical question, Jesus quoted Psalm 118:22–23. The stone the builders rejected, representing the Son, has become the cornerstone of God's kingdom (compare Acts 4:11; 1 Peter 2:4, 6–7). Traditionally, this phrase refers to a cornerstone buried in the foundation. But it could also mean the capstone at the top of a building, marking its completion. In either case, it was the most important stone.
Although the builders, representing the religious leaders, rejected the stone, their actions still took place according to God’s will (Acts 4:28). God used even their evil intentions to accomplish his plan, which is wonderful to see. Those who put Jesus to death became instruments in God’s purpose for the Son of Man "to give His life as a ransom for many" (Mark 10:45).