During his ministry here on earth, much of the teaching of Jesus was carried out in the form of Q & A.  That is, people would ask him questions and he would give answers.  Sometimes those questions were sincere, as when a person knew that they needed wisdom or understanding.  Sometimes they were designed to catch Jesus in some kind of trap.  This second kind of questioner usually thought they left Jesus with no good way out by the way they framed the question.  They were always wrong.  Jesus inevitably gave an answer that exposed the questioner’s mistaken assumptions.

Sometimes Jesus would ask the questions himself.  Here in Luke 20, we get just such an example, and it is designed by Jesus to lead us into the highest levels of truth about himself.  

⁴¹ But he said to them, “How can they say that the Christ is David’s son? ⁴² For David himself says in the Book of Psalms,

⁴⁴ David thus calls him Lord, so how is he his son?”

His listeners believed that the Christ would be the Son of David, thus making Son of David a messianic title.  What they could not explain is this excerpt from David’s own Psalm 110.  How is the Messiah, or David’s “my Lord” in the psalm, supposed to be the son of David at the same time?

We sometimes hear of cultures who worship or “venerate” their ancestors, such as China.  Most cultures at least intuitively give them some sort of honor.  Our memory of great people in the past seems to automatically make the great people of the present look, by comparison, well, a lot less great.  Sometimes our predecessors even make us look silly and self-centered by their sacrifice and commitment to a cause.

Of course, lately the trend is to bash and expose past heroes for the big phonies we now imagine they really were.  As far as I can tell, this is normally overblown – a way of making yourself look better by making someone else who is presently respected look worse.  The dead can’t defend themselves so they make an easy target for criticism.  Better to just admit that no one is perfect, least of all us, and leave the dead alone.

But Jesus has come across something very different indeed.  How many cultures go so far as to honor their descendants?  I can’t think of one.  Yet Jesus seems to have found a hero of Israel’s past, David, the great king, who is doing precisely that.  He is calling his descendant “my Lord.”  Why?

The answer Jesus alluded to is found explained not here, but in later Christian doctrine.  The teachings of Christianity were not given fully formed by Jesus or even his disciples, but often had to be cobbled together from different passages.  If an idea could hold all the separate notions in tension without destroying or denying any of them, then that idea is the clearest statement of biblically based truth.

So here, the idea is found in the two natures of Jesus.  He is not Son of David only, that is, merely human.  He is also not just the Son of God as in only divine.  He is both at the same time.  The true Christian faith does not hold to one as opposed to the other, but to both.  

I happen to like how this is expressed in the Athanasian Creed.  Here it is in an Anglican wording.

For the right Faith is, that we believe and confess, that our Lord Jesus Christ, the Son of God, is God and Man; God, of the substance of the Father, begotten before the worlds; and Man of the substance of his Mother, born in the world; Perfect God and perfect Man, of a reasonable soul and human flesh subsisting.

Equal to the Father, as touching his Godhead; and inferior to the Father, as touching his manhood; Who, although he be God and Man, yet he is not two, but one Christ; One, not by conversion of the Godhead into flesh but by taking of the Manhood into God; One altogether; not by confusion of Substance, but by unity of Person.

See https://www.anglicancommunion.org/media/109017/Athanasian-Creed.pdf