Matthew 22:41 Now while the Pharisees were gathered together, Jesus questioned them, (42) Saying, What do you think concerning the Christ? Whose son is He? They said to Him, David's. (43) He said to them, How then does David in spirit call Him Lord.?
From [Matthew] 21:23 to 22:46, during His last visit to Jerusalem, the center of Judaism, Christ was surrounded by the chief priests, elders, Pharisees, Herodians, Sadducees, and a lawyer, who endeavored to ensnare Him by asking puzzling and crafty questions. They knew religion, politics, belief, and the law, but they paid no attention to Christ.
Hence, He asked them, “What do you think concerning the Christ?” This question of questions must be answered by everyone. Just as in ancient times, people today care for [other] things, not for Christ. They simply have no concept concerning Him.
But God's concern is for Christ, and Christ's concern is for Himself. Hence He asks "What do you think about Christ? Whose Son is He?" This question touches Christ's Person, which is a mystery, the most perplexing matter in the universe.
When the Pharisees were asked this question by the Lord, they replied that Christ was David's son (v. 42). No doubt, according to the Scriptures, this answer was correct.
Then the Lord said, “How then does David in spirit call Him Lord, saying, The Lord said to My Lord, Sit on My right hand until I put Your enemies underneath Your feet? If then David calls Him Lord, how is He his Son?” (vv. 43-45).
The question here is how a great-grandfather could call his great-grandson Lord. This was one question the Pharisees did not know how to answer. As God, in His divinity, Christ is the Lord of David; as a man, in His humanity, He is the Son of David.
The Pharisees had only half the scriptural knowledge concerning Christ's Person, that He was the Son of David according to His humanity. They did not have the other half, concerning Christ's divinity as the Son of God.