Most of the false disciples '...went back, and walked no more with Him' (v.66). While this is true, it did not necessarily apply to someone like Judas. Physically speaking he was with Christ and the others, but spiritually speaking he was an unregenerate man. Recently, some folks made a big to do with the finding of the 'gospel of Judas.' I heard on the radio some people actually trying to make a case that Judas was a victim, and that he was actually saved. We don't need any deep exegesis to know whether Judas was saved or not. Two verses about Judas is quite revealing enough. In Matthew 26:24 Jesus talks about how it would be better that he would not have been born at all. Also in John 17:12 Jesus spoke of his 'lost' condition in contrast to the others that He kept, He also calls him the 'son of perdition.' We don't even need to go to other verses at all since in out text Jesus describes him as 'a devil.'
What was significant about this is that Judas stayed with Jesus and yet he isn't saved. I believe a greater judgment awaits the one who, like Judas, hangs around the things of God, who outwardly conform to religious activities - and yet inwardly there really is no genuine saving repentant faith.
Jesus taught that Capernum will suffer greater judgment than that of Tyre and Sidon (see Luke 10:15). And why? because they have received so much revelation from God through Jesus Christ, that they will be held responsible for. Ironically, God's heaviest judgment does not fall on the overtly pagan, but rather on the false disciples. It is interesting also to mention that chapter 6 took place in Capernum (see verses 24, and 59).
The application for us is both serious and scary. It could very well be that the unregenerate man who weekly attends church, who have been exposed to Biblical truths, and have rejected the convicting element of the Holy Spirit to get saved, that he will indeed suffer the greater judgment of God. The same would certainly be true of the unsaved child who grew up in a Christian home who is negligent of Salvation. Perhaps an unsaved Bible College student, or this could also be said especially of an apostate evangelist or an unsaved Bible preacher (see also James 3:1). How ironic, but how true. Ultimately, you either have The Lord Jesus Christ as Savior or you don't.