"God forbid" meant "this must not happen." Jesus answers by pointing them to Scripture. If the rejection of the son could never happen, how would they explain what is written in Psalm 118:22? The word "then" ties the question to their protest. Before He spoke, Jesus "beheld them," which Gill describes as "Looked very earnestly and wistly at them" (wistly is an old word for intently). Matthew Henry says, "He beheld them, to see if they would blush at their own folly." The question presses them to see that the builders in the psalm stand where the husbandmen stood, and that God would honor the One they rejected.

Jesus does not wait for a reply. He goes on to say what the stone will do: "Whosoever shall fall upon that stone shall be broken; but on whomsoever it shall fall, it will grind him to powder" (Luke 20:18). Barnes, on the parallel in Matthew 21:44, says those who reject Him and continue impenitent "shall be crushed by him in the day of judgment, and perish forever." The chief priests and scribes answered with action, not words: they "the same hour sought to lay hands on him; and they feared the people: for they perceived that he had spoken this parable against them" (Luke 20:19).

Jesus quotes Psalm 118:22, which in the KJV of the psalm reads "the stone which the builders refused is become the head stone of the corner." The head of the corner was the main stone of a building's corner. Barnes, writing on Matthew 21:42, says the stone "represents the Lord Jesus, proposed to the Jews as the foundation or cornerstone on which to build the church, but rejected by them."

The Greek verb for "rejected," apodokimazo, carries the sense of disapproving or repudiating. The word for "beheld," emblepo, means to look at someone directly, which is why the ESV says Jesus "looked directly at them." Gill identifies the builders as the principal men among the Jews, in both church and state.

This question shows how God overturns human verdicts. The builders looked at the stone and threw it aside. God took that same stone and made it the most important stone in the building. The parable had ended with a son cast out and killed. The psalm shows that this would not be the end of His story.

The apostles later preached this same verse about Jesus. Peter told the "rulers of the people, and elders of Israel," "This is the stone which was set at nought of you builders, which is become the head of the corner" (Acts 4:8-11), and he later wrote that to those who believe, He is precious (1 Peter 2:7). What the leaders rejected, God chose.