20:41-44 What was Jesus' purpose in asking this question?

This is also recorded in Mt 22:41-45 and Mk 12:35-37. In asking this question Jesus was testing the Pharisees' knowledge of Messiah (cp Mt 22:41-45). The Pharisees' answer reflected their view that Messiah would be no more than a man, and Jesus' reply was another assertion of His Deity.

When the Pharisees said that Messiah would be David's son, meaning that He would be a human descendant of King David, Jesus quoted Psa 110:1 to show them that He would not be a mere human, because David addresses Him as Lord - kurios, God (cp Psa 110:1-7).

Jesus was not disputing that Messiah would be the son of David (cp Mt 1:1), but that being called the son of David does not sum up all that Messiah is. He is also God (cp Ac 2:25-36).

See also comments on Mt 1:18-21, 3:16-17; Lk 1:35 (B); Jn 1:1, 5:16-23, 12:41; Ac 13:33, 20:28; Php 2:5-8; Col 2:9; 1Ti 3:16; He 1:5, 5:5; 1Jn 5:6-9; Rev 1:8