"And in mercy shall the throne be established: and he shall sit upon it in truth in the tabernacle of David, judging, and seeking judgment, and hasting righteousness." (Isaiah 15: 5)
Five is the number of grace, which one can also call "mercy".
The throne of any king is established by God's grace, for all power comes from God (Romans 13: 1)
God invites us to be established in His grace:
"Be not carried about with divers and strange doctrines. For it is a good thing that the heart be established with grace; not with meats, which have not profited them that have been occupied therein." (Hebrews 13: 9)
God the Father has seated us in His Son (Ephesians 2: 4-6), and He sees us in Christ, thus we are made sons of God (1 John 3: 1-3)
Because of what Jesus has done, we are both kings and priests:
"But ye are a chosen generation, a royal priesthood, an holy nation, a peculiar people; that ye should shew forth the praises of him who hath called you out of darkness into his marvellous light." (1 Peter 2: 9)