Adam Clarke's Commentary on the Bible

Verse 15 (Matthew 23:15)

Compass sea and land - A proverbial expression, similar to ours, You leave no stone unturned; intimating that they did all in their power to gain converts, not to God, but to their sect. These we may suppose were principally sought for among the Gentiles, for the bulk of the Jewish nation was already on the side of the Pharisees.

Proselyte - Προσηλυτος , a stranger, or foreigner; one who is come from his own people and country, to sojourn with another. See the different kinds of proselytes explained in the note on Exodus 12:43 ; (note).

The child of hell - A Hebraism for an excessively wicked person, such as might claim hell for his mother, and the devil for his father.

Twofold - the child of - The Greek word διπλοτερον , which has generally been translated twofold, Kypke has demonstrated to mean more deceitful. Απλοῦς is used by the best Greek writers for simple, sincere, απλότης for simplicity, sincerity; so διπλοῦς , deceitful, dissembling, and διπλόη , hypocrisy, fraudulence, and διπλοτερον , more fraudulent, more deceitful, more hypocritical. See also Suidas in Διπλοη .

Dr. Lightfoot, and others, observe, that the proselytes were considered by the Jewish nation as the scabs of the Church, and hindered the coming of the Messiah; and Justin Martyr observes, that "the proselytes did not only disbelieve Christ's doctrine, but were abundantly more blasphemous against him than the Jews themselves, endeavoring to torment and cut off the Christians wherever they could; they being in this the instruments of the scribes and Pharisees."

- Adam Clarke's Commentary on the Bible