Because He hath appointed a day, in [the] which He will judge the world in righteousness by [that] man whom He hath ordained; [whereof] He hath given assurance unto all [men], in that He hath raised Him from the dead.
If the question was raised, why is Christ the righteous Judge? This is because who Christ is; the only begotten of God the Father who died on the cross paying man's sin-debt. Christ is the righteous judge being that He was slain from the foundation of the world to be man's propitiator. Christ is the righteous judge it is in Him deity and humanity met. Christ is the righteous judge in that He was the first fruit of the dead who was raised with all power in His hands in heaven and earth.
Being human and divine, Christ met all of the Father's requirements to redeem man back to Himself. Being human and divine, Christ could relate to His Father, humanity and the world. Christ was fully God and man. This is because He has God the Holy Spirit as His Father and was born of the Virgin Mary who was fully human.
It worth noting that Christ was in the beginning with God and all things were created by Him (John 1:1-4). It is in Christ the Word was made flesh and dwelt among humanity (John 1:14).
These questions come to mind, who would you prefer as the righteous judge, Christ or man? Would you prefer a judge who is righteous and changes not or someone that is sinful by nature? Christ is the second person in the Godhead, who is God and knows all, sees all and can do all.
Christ is the righteous judge who is coming back to judge the world with His truth and His eternal justice.
Comments are welcomed.