It is good for us to try to understand how the first readers might have understood an Old Testament passage before going on to the New Testament understanding of that passage.
Possible meaning when written
At first, Psalm 110 may have been understood as a royal enthronement poem. When David had captured Jerusalem, it would have been natural to have a ceremony as he moved his capital there. Within that ceremony it might have been expected that someone, perhaps a court poet, would have recited something fitting for the occasion. The poet might include something honouring the military victory, honouring the Lord for granting it. He would have been likely to include something indicating that this is just the beginning of what the Lord will do for David. The poet might include something connecting the ancient narrative of Melchizedek, King of Salem with what has just happened (Gen 14:18). David has just conquered the city where Melchizedek had once been king.
Through the years of David’s reign, this understanding of the words may have been satisfactory to the people. The kingdom was continually expanding, with frequent victories. While David was forbidden from entering the holy place, being not of the Levitical priests, he did function at times in a priestly way, interceding and offering certain sacrifices (2 Sam 24:25).
Post-exilic use
By the post-exilic era, this understanding of the psalm would have been difficult, if not impossible for the pious Jew. The house of David had fallen. The promise of the psalm seemed to have been broken. A new understanding was needed if the psalm was to be maintained. Perhaps at that time it began to occur to the faithful Jews that much of what had been said, which they had taken as applying to David, did not really apply to David himself but to some later descendent of David.
It may have been in this context that messianic interpretations of this psalm became commonly accepted. Doubts that may have lingered in the back of people’s minds about the things ascribed in the psalm — if applied to the kings they knew of David’s line — can now be cleared up by ascribing these things to the coming Messiah. But not all the questions are answered by the messianic interpretation, at least as they understood the Messiah.
Jesus implies a messianic interpretation that indicates the incarnation of the Messiah. Many Christians, when reading Matthew 22:42-45 (and Mark 12) have accepted this but perhaps have not fully explored the thought.
More than
Any reconstructed history of this psalm is, of course, speculative. We do not know that it came out of the moving of David’s capital to Jerusalem or know the details of the intertestamental understanding of the psalm.
What we do know is that Israel possessed, for centuries, a statement that they took to mean something, but which meant more than they realized — likely even more than the original author realized (cf Dan 12:8; 1 Peter 1:10-12).
We conclude, therefore, that this psalm teaches us more than that God knows what he is going to do a long time before he does it. It also teaches us that there is more in scripture than we tend to realize. It teaches that there may sometimes be multiple levels of meaning, some of which are attainable by humans, others of which refer only to the Christ. It also teaches us that God can work with our imperfect understandings, as with our other imperfections.
Lastly, this psalm reminds us of that wondrous fact of the incarnation. That men want to be treated as gods, we know. That God would become man to die for his rebellious creatures is a wonder we have not sufficiently pondered or communicated.
“Now to him who is able to do far more abundantly than all that we ask or think, according to the power at work within us, 21to him be glory in the church and in Christ Jesus throughout all generations, forever and ever. Amen” (Ephesians 3:20-21).