Matthew 23:15

Twofold more a son of hell than yourselves (uion geennh diploteron mwn). It is a convert to Pharisaism rather than Judaism that is meant by "one proselyte" (ena proshluton), from prosercomai, newcomers, aliens. There were two kinds of proselytes: of the gate (not actual Jews, but God-fearers and well-wishers of Judaism, like Cornelius), of righteousness who received circumcision and became actual Jews. But a very small per cent of the latter became Pharisees. There was a Hellenistic Jewish literature (Philo, Sibylline Oracles, etc.) designed to attract Gentiles to Judaism. But the Pharisaic missionary zeal (compass, periaghte, go around) was a comparative failure. And success was even worse, Jesus says with pitiless plainness. The "son of Gehenna" means one fitted for and so destined for Gehenna. "The more converted the more perverted" (H.J. Holtzmann). The Pharisees claimed to be in a special sense sons of the kingdom ( Matthew 8:12 ). They were more partisan than pious. Diplou (twofold, double) is common in the papyri. The comparative here used, as if from diplo, appears also in Appian. Note the ablative of comparison hmn. It was a withering thrust.