To be the Devil's Advocate Matthew 23:13 does not definitely say (IMO) those who are being kept back will never go in.
(Agreed)
The Bible in Basic English:
Matthew 23:13 But a curse is on you, scribes and Pharisees, false ones! because you are shutting the kingdom of heaven against men: for you do not go in yourselves, and those who are going in, you keep back (or "you hinder").
Then in the KJV in a parallel passage:
Luke 11:52 Woe unto you, lawyers! for ye have taken away the key of knowledge: ye entered not in yourselves, and them that were entering in ye hindered.
To hinder is not the same as being forever shut out.
(Agreed, being blocked at a point in time does not address what might happen down the road.)
So Irresistible Grace should probably be upgraded to Ultimate Irresistible Grace TULUP
.