New Defender's Study Bible Notes
6:66 went back. It is possible, therefore, to be a “disciple” of Christ yet not a true believer, for these disciples would not have gone back otherwise (note John 6:39; also I John 2:19). It may be only coincidence, since the verse divisions were not in the original autographs, but it is at least worth noting (in light of the significance of the number 666—see Revelation 13:18) that this verse, John 6:66, which marks those disciples who were false disciples, is the only “6:66” verse in any of the twenty-seven books of the New Testament. Note also John 6:70.