Aaron was a priest and David was a king, but Jesus was both. God told the prophet Nathan that David’s kingdom would have a significant future (2 Samuel 7:5-17). The kingdom of David would be one that would last forever, but this could not mean that David would be king forever, since he was going to die (2 Samuel 7:12).

We gain more insight into this passage as we think of the promise given to Eve and the covenant given to Abraham. Up until this point, God had protected the line to the Seed (Christ), and had established His covenant with Abraham that through his descendants the whole world would be blessed. This covenant was re-established with Moses and others that God would be their God and they would be His people. In this passage, David, as a King in the line of Abraham, received promises that God would establish his kingdom, and his throne would be established forever .

We read something significant about Jesus Christ in the book of Hebrews that relates to both the priests and kings of Israel. Citing Psalm 110:4, Hebrews 7:17 tells us that Jesus Christ is a priest forever in the order of Melchizedek - the king and priest of Salem (Genesis 14:18). This means that Jesus was both a King and Priest for His people.

The significance comes when we see the extent to which Jesus is both King and Priest. Jesus does not need to officiate at sacrifices, but offered Himself as a sacrifice in our place (Ephesians 5:2), and in doing so, satisfied God’s justice. Jesus is the ultimate Priest because in Him we have direct access to the Father, and can boldly approach the throne of grace (Hebrews 4:16). The rending of the temple’s curtain at Christ’s death symbolizes these truths.

Jesus is also the ultimate King. He rose from the dead, conquered both sin and death, and is now seated on His throne at the Father’s right hand (Acts 2:33). No other human could have possibly done this, but Jesus, as the risen Lord, has total authority (Matthew 28:18). He is the great King.

Aaron cannot claim the ultimate priesthood, since his priesthood required ongoing sacrifices for sin. David cannot claim the ultimate kingship, since he died and his rule died with him. However, Christ is both the ultimate Priest and King, and His priesthood and rule extend forever.

Please notice that God has established and re-established His promise of good news in our Savior Jesus Christ through men, nations, priesthoods, and kingdoms. He has protected the line to the Seed of promise, established His people, shone His light on our sin, and reminded us over and over again of His grace in salvation through the Messiah. The Old Testament reveals that God repeatedly let His people know that He would send the Messiah to redeem those who would trust in Him.  http://www.answersingenesis.org/articles/2010/08/04/consistency-in-priesthood