If a disciple is 'a follower', but then stops following, they are no longer a disciple. This indicates that merely claiming to be a follower of Jesus is insufficient to prove being a Christian because some disciples stop following, as here, due to not actually believing as a Christian should, or what a Christian should. This raises intriguing thoughts about gaining salvation.
Does the text mention – or even hint at – salvation? Were they saved from what we – in the 21st century – suppose salvation saves believers from? Or was Jesus speaking about something different? The entire chapter needs to be read, to grasp the context and to see that Jesus was speaking about being given everlasting life at the point of truly believing in Jesus and ‘partaking’ of him, by faith.
Chapter 6 starts with the miraculous feeding of the 5,000 with two small fishes and five barley loaves. The next day the crowd that sought him out were told by Jesus not to seek him for literal bread but for that which endures to eternal life. They should believe in the one God had sent (himself). People then referred to the miracle of manna in the wilderness – bread from heaven. Jesus put them right. It wasn’t Moses who gave their forefathers bread from heaven, “but my Father gives you the true bread from heaven. For the bread of God is he which cometh down from heaven, and giveth life unto the world” (vss 26-35). Himself.
That was the point of their unbelief which caused them to fail to understand what he was teaching, which led to them taking offense at then saying they needed to come to him to ‘partake’ of him as the bread of life and to ‘drink’ of him, that they might never thirst again (reminiscent of John 4:10-14 – Christ giving living water to those worshipping the Father in spirit and in truth.)
Jesus said ten times that those who did that would be given everlasting life. (Jn 6:35; 37; 39; 40; 44; 47; 51; 54; 56 & 57.) That life eternal would start at the point of true belief in who Jesus truly is, and go on. Such ones would never be cast out. But Jesus addressed those who had seen him that they did not have true belief in him – they had not come to him. Christ’s promise in chapter 6 is that, “Every one which seeth the Son, and believeth on him, may have everlasting life, and I will raise him up at the last day” (vs 40).
Clearly, those disciples (followers) who murmured at his further teaching on ‘partaking’ of him, did not believe on him despite having seen him and his miracles, and so they stopped following. Thereby, they proved that the Father had not drawn them to the Son, that they might have everlasting life (vs 44). That is what chapter 6 is all about. Once clarity dawns on this matter of everlasting life, and what this particular belief of chapter 6 is, then we may progress to see what other sections of scripture say about salvation and so built up a more detailed picture.
The answer is that following Jesus and learning about him makes a person a disciple, but not all disciples continue following and learning to the point where they truly believe in Jesus for who he actually is (vss 66 - 69). Only those who do are given the gift of everlasting life, the moment they truly believe – “He that believes on me HAS everlasting life” – vs 47.