It may be asked
                                Can this Jesus be King seeing he was crucified nearly 2000 years ago?

To which the Bible replies:-
Jesus of Nazareth, a man approved of God among you (Jews) by miracles and wonders and signs, which God did by him in the midst of you, as ye yourselves also know:
Him, being delivered by the determinate counsel and foreknowledge of God, ye have taken, and by wicked hands have crucified and slain:
Whom God hath raised up, having loosed the pains of death Acts 2:22-24
Again:-
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.  Acts 17:31

What proof is there that this is true?

First and foremost that God has declared it. In addition He has provided ample historical and present day evidence.

Some will say  -- There is no God

Psalm 14:1 speaks of these:- The fool hath said in his heart, There is no God.

When will Christ the King come?

Jesus spoke of