Ps 110:1-2, Lk 1:31-33, Mt 3:16-17, Mt 21:9, Eph 1:20-21, Phil 2:8-11

In the Old Testament, God told David that he would set up an eternal king as a descendant of David. (1 Chronicles 17:11-14)

1 Chronicles 17:11 And it shall come to pass, when thy days be expired that thou must go to be with thy fathers, that I will raise up thy seed after thee, which shall be of thy sons; and I will establish his kingdom. 12 He shall build me an house, and I will stablish his throne for ever. 13 I will be his father, and he shall be my son: and I will not take my mercy away from him, as I took it from him that was before thee: 14 But I will settle him in mine house and in my kingdom for ever: and his throne shall be established for evermore.

In the Old Testament David saw God granting Christ kingship and giving Christ dominion over his enemies. (Psalm 110:1-2)

Psalm 110:1 The Lord said unto my Lord, Sit thou at my right hand, until I make thine enemies thy footstool. 2 The Lord shall send the rod of thy strength out of Zion: rule thou in the midst of thine enemies.

As a descendant of David, Christ the King has come. That Christ is Jesus. (Luke 1:31-33, Matthew 1:16)

Luke 1:31 And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name Jesus. 32 He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David: 33 And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end.

Matthew 1:16 And Jacob begat Joseph the husband of Mary, of whom was born Jesus, who is called Christ.

When Jesus entered Jerusalem, the people welcomed him as a king who had come as a descendant of David. (Mt 21:9)

Mt 21:9 And the multitudes that went before, and that followed, cried, saying, Hosanna to the son of David: Blessed is he that cometh in the name of the Lord; Hosanna in the highest.

Jesus is the true King, the Christ, who has received an eternal throne from God. (Ephesians 1:20-21, Philippians 2:8-11)

Ephesians 1:20 Which he wrought in Christ, when he raised him from the dead, and set him at his own right hand in the heavenly places, 21 Far above all principality, and power, and might, and dominion, and every name that is named, not only in this world, but also in that which is to come:

Philippians 2:8 And being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross. 9 Wherefore God also hath highly exalted him, and given him a name which is above every name: 10 That at the name of Jesus every knee should bow, of things in heaven, and things in earth, and things under the earth; 11 And that every tongue should confess that Jesus Christ is Lord, to the glory of God the Father.