Jesus had just defeated the scholarly Sadducees in a verbal battle of wits. In doing so, he had caused the Pharisees to rejoice, in that Jesus had shown how the Law points to the resurrection, something the rabbis had heretofore been unable to do. Therefore, perhaps not to appear he supported this group over that of the Sadducees, Jesus asked the disciples of the Pharisees how their teachers (the rabbis / scribes – see Mark 13:35) taught the Messiah was David’s son (Matthew 22:41; Luke 20:41).
Jesus pointed out two things concerning David and the Messiah. First, David referred to the Messiah as his Lord (H113; Psalm 110:1), but in the text at Psalm 110:4 it is adonai (H136), which is another name the Jews used for God. In referring to the Messiah as his Lord (Luke 20:42-43) David was admitting he was greater than himself. This was quite out of place in that culture. The father was always greater than the son. Abraham was greater than Isaac, and Isaac was considered greater than Jacob.
The disciples of John thought that he should be greater than Jesus, whom he had baptized (cf. John 3:26), but John testified that Jesus was greater than he, even though John had baptized him (cf. Matthew 3:13-15). The reason John claimed Jesus was greater was that Jesus lived before John (cf. John 1:15, 27, 30). In other words, Jesus, although born six months after John, existed before John. This could be so only if Jesus were a divine being.
This is also the point of view taken by David in Psalm 110. David reasoned, as he spoke in the Spirit, that, if the LORD (Jehovah) had told the Messiah to sit at his right hand, i.e. upon God’s throne (Psalm 110:1), then the Messiah had to, at the very least, be a heavenly being come to earth through David’s lineage, because no mere man has ever or will ever legitimately sit upon God’s throne.
If David referred to the Messiah as Lord, how could he be David’s son (Luke 20:44)? Jesus’ logic is sound, because, not only does the Messiah sit upon David’s throne, but also upon God’s throne (Psalm 110:1, 4; cf. Revelation 3:21; 22:1, 3). Moreover, the Messiah cannot be merely any heavenly being, but he must be God come in flesh, because God tells us through Isaiah that he will not share his glory with anyone (Isaiah 42:8; 48:11). There is no god with God (Deuteronomy 32:39), and speaking in terms of men, God claimed there was no god formed before him, nor was there any formed after him (Isaiah 43:10). In other words, God, who knows all things, knows of no other God besides himself (Isaiah 44:8). Therefore, the Messiah, who shares the glory of God, must also be God come to earth in flesh.
In this Psalm the One who sat upon God’s throne at the right hand of God, was to come to earth and rule among the nations (Psalm 110:2, 6). It is the most quoted Psalm in the New Testament (cf. Acts 2:34; 7:56; Romans 8:34; 1 Corinthians 15:25; Ephesians 1:20; Colossians 3:1; Hebrews 1:3, 13; 5:6; 7:17, 21; 8:1; 10:12-13; 1 Peter 3:22; Revelation 3:21), and Jesus means to show those who listened that the Messiah, whom they claimed to be David’s son, was also the very Son of God come in the flesh, and he, Jesus, is that One, and the Jewish authorities planned to kill him (Luke 20:14-15).