but at least by the time of Matthew 21, the population of the kingdom of God is non-zero. harlots and thieves are already entering.
per John 3, no one enters without being born from above.
this is also explained in the next verse:
For John came to you in the way of righteousness, and you did not believe him; but tax collectors and harlots believed him; and when you saw it, you did not afterward relent and believe him.
(Matthew 21:32)
per v. 31 these harlots and tax collectors who believed the message of John, which is the message of Elijah, turning the hearts of the children to their parents and the parents to their children, these are already entering. ((Malachi 4:5-6))
this is also explaining that the chief priests and elders are not entering because they did not believe him even though they saw it -- saw what? saw these sinners entering?
also in Matthew 21:
Jesus said to them, “Have you never read in the Scriptures:
‘The stone which the builders rejected
Has become the chief cornerstone.
This was the Lord’s doing,
And it is marvelous in our eyes’?
Therefore I say to you, the kingdom of God will be taken from you and given to a nation bearing the fruits of it, and whoever falls on this stone will be broken; but on whomever it falls, it will grind him to powder.”
(Matthew 21:42-44)
He tells the chief priests and elders, the kingdom of God will be taken away from them and given to a people bearing fruit of it.
that presupposes that the kingdom of God is with them, that it could be taken away! how was it theirs, to be taken away? when did it become theirs? is it taken away? were they baptized, then acquiring it? baptized into Moses? if that is the case, the kingdom of God has been non-empty for a thousand years before Christ came, populated by those who were counted righteous before Him.
can anyone enter the kingdom without being born from above? John 3 says, no. so for example, David, or Daniel - are they in the kingdom, have they already been in the kingdom? if so, they were born from above. when and how?
and all this is before the crucifixion, before the pouring out of the Spirit at Firstfruits, called Pentecost.