It was only at Pentecost that the gift of the Holy Spirit was given to believers (Acts 2). And it is only the Holy Spirit who baptizes believers into the Body of Christ (1 Cor 12). And since the Church is the Body of Christ, the Church began on the day of Pentecost.
Subsequently the "mystery" of the Church was revealed to Paul. It had been hidden from all the OT prophets as Paul tells us plainly. Thus the OT saints had to wait until Pentecost to receive the gift of the Spirit. And unless Christ had finished His redemptive work and ascended to Heaven, the Holy Spirit could not have been poured out upon earth. So you seem to have forgotten all of this in trying to justify the existence of the Church in the OT.
“And it is only the Holy Spirit who baptizes believers into the Body of Christ”
Amen probably why John the Baptist was filled with the Holy Ghost from birth.
“But the angel said unto him, Fear not, Zacharias: for thy prayer is heard; and thy wife Elisabeth shall bear thee a son, and
thou shalt call his name John. For he shall be great in the sight of the Lord, and shall drink neither wine nor strong drink; and
he shall be filled with the Holy Ghost, even from his mother's womb.
And he shall go before him
in the spirit and power of Elijah , to turn the hearts of the fathers to the children, and the disobedient to the wisdom of the just; to
make ready a people prepared for the Lord.”
Luke 1:13, 15, 17 KJV
in the question Jesus asked “
Was johns baptism from man ? Or from God ? “
I’d say it was from God in the womb where the spirit of baptism begins in the “baptizer John “ who was come to prepare hearts and minds for Jesus through baptism of repentance and remission of sins even before his name had been disclosed