Charles Spurgeon Commentary Matthew 23:15

"Woe unto you, scribes and Pharisees, hypocrites! for ye compass sea and land to make one proselyte; and when he is become so, ye make him twofold more a son of hell than yourselves." — Matthew 23:15 (ASV)

The third “woe” related to the unholy zeal of the scribes and Pharisees in gaining adherents to Judaism and their own party, and by the process making them even worse than themselves. They freely gave time and trouble to the work with the prospect of a very slight return, Ye compass sea and land to make one proselyte. They would, as it were, drag the Great Sea with a seine net in the hope of entangling one proselyte in its meshes, or they would go over all the land in order to persuade one Gentile to be circumcised so as to become a Jew outwardly.

The result to the proselyte was only evil: When he is made, ye make him twofold more the child of hell than yourselves. The corrupted usually become bigots. The proselyte would naturally imitate the vices of his hypocritical teachers, without having that knowledge of the Scriptures which might to some extent exercise a wholesome restraint upon them. The circumcised heathen would be a Judas rather than a Jew, a veritable son of perdition.