Was Judas ever a believer?
Judas was never a true follower of Jesus. His only god was money. In fact, the only other time the phrase "son of perdition" is used to refer to anyone other than Judas is in reference to the Antichrist, the sworn enemy of every believer in the end times.
Judas was chosen by Jesus. Jesus knew what Judas would do. He had every chance, like the other disciples, to be saved. If had been, Jesus still would have found a way to accomplish His sacrifice. But, Jesus knew from the beginning that Judas would be against him.
Also, the Bible says that after the Last Supper, the devil entered Judas. Believers can be oppressed by the devil, but they can't be possessed by him. That's because, God, in the form of the Holy Spirit, resides in each believer. "Did I Myself not choose you, the twelve, and yet one of you is a devil? Now He meant Judas the son of Simon Iscariot, for he, one of the twelve, was going to betray Him" (John 6:70-71).