Matthew 23:13-15

13 “But woe to you, scribes and Pharisees, hypocrites! For you shut up the kingdom of heaven against men; for you neither go in yourselves, nor do you allow those who are entering to go in.

14 Woe to you, scribes and Pharisees, hypocrites! For you devour widows’ houses, and for a pretense make long prayers. Therefore you will receive greater condemnation.

15 “Woe to you, scribes and Pharisees, hypocrites! For you travel land and sea to win one proselyte, and when he is won, you make him twice as much a son of hell as yourselves.

Christ had preached that He was the way into the kingdom. By rejecting Christ (v.13), the Pharisees were closing the kingdom to any who followed them.

False religion and pretense are always the worst enemies of the truth and are far more dangerous than immorality or indifference. As the religious leaders of the Jews, they were held guilty before God of blocking the way for others seeking to enter into the kingdom of God. — Walvoord, page 172

Verse 14 isn’t in most manuscripts. It was probably copied from Mark 12:40 and Luke 20:47.

The Jews recognized two sorts of proselytes [converts from paganism to Judaism]: those who agreed to the so-called seven precepts of Noah, and those who submitted to circumcision and became full Jews by religion. — KJV Commentary, page 1220.

The Pharisees were described as extremely energetic on both land and sea to make proselytes of the Jewish religion. But when they were successful, Jesus Charged, “Ye make him twofold more the child of hell than yourselves,” In referring to hell, Christ used the word Geenna or Gehenna, a reference to eternal damnation, rather than to Hades, the temporary abode of the wicked in the intermediate state. The Pharisees and their proselytes both would end up in eternal damnation. — Walvoord, page 172.