Matthew 23:14-15 Woe unto you, scribes and Pharisees, hypocrites! for ye devour widows' houses, and for a pretence make long prayer: therefore ye shall receive the greater damnation. Woe unto you, scribes and Pharisees, 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.
v. 14 The greater damnation
Degrees of punishment in hell is what I was taught concerning "the greater damnation". While that could be interpreted that way, there is a simpler possible comparison. Jesus condemns the hypocritical Pharisees, who made much religious show with their long prayers and such. But behind their religious façade lay hearts of evil which had no compassion even for the helpless, such as the widows. If they had the advantage, they would gladly devour their homes, their possessions, their all. In their weakest time with little defense, these hypocrites take advantage of widows. Perhaps, like some modern religions, they offer their prayers to obtain their substance. Whatever damnation they laid on the widows, theirs will be greater. Not that their eternal punishment will be greater than the eternal punishment of others, but what happens to them is greater and more severe than what they did to the helpless widows. Vengeance is God's; He will repay. Psalm 68:5 A father of the fatherless, and a judge of the widows, is God in his holy habitation.
v. 15 Twofold more the child of hell
Just because one is a Jewish proselyte does not guarantee that one is bound for eternal punishment. Some proselytes gladly received the message of the apostles (e.g. Acts 2:10, 3:5; 13:43). So I don't see how it fits that this is referring to the destiny of the proselyte. They are not "two-fold" more children of hell in their destiny, but in their actions. The converts of the Pharisees learn their teachings, mimic their ways, and accentuate their flaws. So, what is bad in the teacher is worse in the student. Though Saul was not strictly a proselyte (if I understand the term), he was a disciple of the Pharisee Gamaliel. Gamaliel is presented as a fairly reasonable man who opposed killing Christians, prefering to let Christianity die out of its own accord (he hoped). Yet his disciple Saul was exceedingly mad against them. Where Gamaliel was willing to let them slowly die out as a Jewish sect, Saul just wanted them to die. He was quite successful in exterminating many of them (Cf. Acts 5:34-40; 22:3-4; 26:9-11). Is not this what Jesus meant?