Let me disagree respectfully. Christianity was not just “building on a fertile ground.” The religion of the Old Testament - better put, the faith (objectively speaking) of the Old Testament - was not something different than Christianity. It was pre-Christianity. Or, better, Christianity is the Old Testament faith brought to full flower, the Pharisees and Sadducees notwithstanding. They were the errorists. Joseph and Mary, Zechariah and Elizabeth, Simeon and Anna, were the faithful, the true children of Abraham, people after the heart of God even as King David was.
Are we talking degrees here? I think, the pivot was "Suffer children to come to me, and forbid them not" (Luke 18:16). I agree that in the role that the children play in the Old Testament there is a precursor to that. Surely, the scenes of depravity commonplace in pagan Rome would not be possible in Jerusalem, but the idea that there is something mystical about the soul of a child (Matthew 18:10) had a unique impact on the Christian culture, as the author notes.