Hebrews 6:4 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
5 And have tasted the good word of God, and the powers of the world to come,
6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
The words "if they shall fall away" scares many believers, who, like the apostles at the Last Supper, keep asking "Is it I, Lord?"
Matthew 26:22 And they were exceeding sorrowful, and began every one of them to say unto him, Lord, is it I?
23 And he answered and said, He that dippeth his hand with me in the dish, the same shall betray me.
24 The Son of man goeth as it is written of him: but woe unto that man by whom the Son of man is betrayed! it had been good for that man if he had not been born.
25 Then Judas, which betrayed him, answered and said, Master, is it I? He said unto him, Thou hast said.
Clearly, Judas, one of the twelve, was the betrayer, the only one that fell away.
But was he saved?
Clearly he was not, as Jesus called him the "son of perdition".
John 17:12 While I was with them in the world, I kept them in thy name: those that thou gavest me I have kept, and none of them is lost, but the son of perdition; that the scripture might be fulfilled.
In terms of Hebrews 6:4-6 cited above, Judas was once enlightened by Jesus' personal teaching; he tasted of the heavenly gift, and even worked miracles in Jesus' Name; he was made partaker of the Holy Ghost (who worked with the apostles), and so he both tasted the good word of God, and the powers of the world to come, but ... he was never saved and that's why he fell away ... he was the son of perdition, chosen among the twelve that the scripture might be fulfilled.
Jesus called him a devil:
John 6:70 Jesus answered them, Have not I chosen you twelve, and one of you is a devil?
71 He spake of Judas Iscariot the son of Simon: for he it was that should betray him, being one of the twelve.
Satan put the thought of betraying Christ into Judas before Jesus washed the disciples' feet. (John 13:2)
And he went out to carry out his nefarious deed immediately after the Passover Meal. (John 13:30)
Judas never turned to God in repentance. He repented of what he had done and killed himself, wanting to end his pangs of guilt through death.
His was not a godly repentance (like Peter's who denied Jesus).
Matthew 27:3 Then Judas, which had betrayed him, when he saw that he was condemned, repented himself, and brought again the thirty pieces of silver to the chief priests and elders,
4 Saying, I have sinned in that I have betrayed the innocent blood. And they said, What is that to us? see thou to that.
5 And he cast down the pieces of silver in the temple, and departed, and went and hanged himself.
The fruit of that repentance was suicide! Clearly Judas was never saved, and went headlong into hell.