Was Judas ever 'in Christ'? Wasn't he given the gift of miracles including that of raising the dead.
Good answer. My thought, however, is that those who are "in Christ" and who "walk away" either capitulate to the lusts and desires of the flesh or they completely leave the faith by conscious decision--they opt for another course for their life.
Obviously, many genuine Christians fall into the flesh and into sin, and perhaps even choose another course for awhile. But genuine born again Christians can never completely leave their spiritual birth behind. In the end it will drive them to repentance. That happened to me in my adolescence. I was raised in the church, and chose a more liberal course for my life, indulging in the flesh. But my conscience got the best of me, along with the testimony of Christians. I was compelled to repent or live in utter failure and shame.
There are others who have had genuine spiritual experience with God and follow Christ's laws. But they sell their spiritual birthright for a bowl of pottage. These indeed were never truly born again and Saved.
Who knows, but God, what a person's true motives are when they accept and embrace Christ as their Way? They could be doing it because it gives one a good reputation, because it seems to work, because it helps people I like, etc. etc. It may simply help a business person make sales!
The important motive to have, when embracing Christ, is to want to be like him, to have him near, to want to spend eternity in his holy presence. That isn't always the motive for people who come to Christ. They may only want to be healed, like the 9 lepers who never turned back to thank Jesus for the miracle.