The first half of the quotation told what the builders did. This half tells what God did. Jesus wants His hearers to see that the rejected stone's rise is God's own work, not an accident. Gill explains "the Lord's doing" as "the exaltation of the Messiah, after he had been so ill treated, and at last put to death by the Jews." The question presses the leaders: if God Himself raises up the stone they reject, what are they really opposing?

The leaders understood the point. Mark says "they knew that he had spoken the parable against them." They wanted to seize Him, but "feared the people," so "they left him, and went their way" (Mark 12:12). They did not give up, though. Mark's next words show them sending Pharisees and Herodians "to catch him in his words" (Mark 12:13).

Jesus quotes Psalm 118:23, which the KJV renders "This is the LORD'S doing; it is marvellous in our eyes." In Mark's Greek the line begins para Kyriou egeneto, literally "from the Lord this came about." The word for "marvellous" is thaumaste, something to be wondered at.

This psalm was already in the air that week. When Jesus rode into Jerusalem, the crowds cried, "Blessed is he that cometh in the name of the Lord" (Mark 11:9), words from the same psalm (Psalm 118:26).

This question shows God's hand in the very rejection the leaders were planning. What looked like the end of the son in the parable becomes the occasion for God's greatest work. Barnes explains "marvellous in our eyes" as "Wonderful in the sight of his people": that God would choose His only Son, let Him be despised and put to death, then raise Him and build His church on Him.

Matthew Henry's Concise Commentary carries the thought into the heart: "the exaltation of Christ was the Lord's doing; and it is his doing to exalt him in our hearts, and to set up his throne there." The same God who lifted the rejected stone is the One who opens eyes to see it with wonder.