Not being able to be un-sonned isn't really far enough for me to know that repentence is a one time thing.
If it was just required one time and after that these people became children of the kingdom, then wouldn't it follow they must have did something wrong to be cast out into outer darkness?
Matthew 8
10When Jesus heard
it, he marvelled, and said to them that followed, Verily I say unto you, I have not found so great faith, no, not in Israel.
11And I say unto you, That many shall come from the east and west, and shall sit down with Abraham, and Isaac, and Jacob, in the kingdom of heaven.
12But the children of the kingdom shall be cast out into outer darkness: there shall be weeping and gnashing of teeth. 13And Jesus said unto the centurion, Go thy way; and as thou hast believed,
so be it done unto thee. And his servant was healed in the selfsame hour.