"Twice the child of Hell" = a convert of the "Rabbis" (teachers of Moses' law & Pharisees)

Matthew 23:15
Woe unto you, scribes and Pharisees ["Rabbis"], hypocrites! for ye compass sea and land to make one proselyte, and when he is made, ye make him twofold more the child of hell than yourselves.
KJV
Matthew 23:15
"Woe to you, teachers of the law and Pharisees, you hypocrites! You travel over land and sea to win a single convert, and when he becomes one, you make him twice as much a son of hell as you are."
NIV