I do not believe this issue here is salvation, but rather a calling by God to be a disciple.
It would appear from the preceding context that the subject is believing on Christ for eternal life or coming to Christ in faith (vv. 35-57).
Isn't verses 51-57 exactly what "some" of the disciples objected to?
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0 ¶ Many therefore of his disciples, when they had heard this,[vv. 51-57] said, This is an hard saying; who can hear it?
61 When Jesus knew in himself that his disciples murmured at it, he said unto them, Doth this offend you?
Didn't Christ already clearly state the very Jews who were his listening audience were unbelievers (Jn. 6:36)?
In a sense, yes, we're all called to be a disciple; but, Jesus specifically called twelve men.
How does that fit with the whole passage beginning in verse 29 right up to verse 57 being directly addressed to unbelieving Jews who rejected what he was teaching (v. 36)? He neither claims them as "his disicples" but clearly identifies them as unbelievers (v. 36) that need to eat his flesh and drink his blood (metaphorically partake of Christ by faith) in order to have eternal life not because they have eternal life and are disciples?
Those called "some" of his disciples cannot possibly be part of that audience he was addressing to believe in him for eternal life which takes up all of John 6:29-57.
Those being addressed as "some" of His disciples are already professed believers in Christ who simply regarded his teaching directed to the unbelieving Jews "too hard" to believe.
60 ¶ Many therefore of his disciples, when they had heard this, said, This is an hard saying; who can hear it?
61 When Jesus knew in himself that his disciples murmured at it, he said unto them, Doth this offend you?
62 What and if ye shall see the Son of man ascend up where he was before?
63 It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life.
64 But there are some of you that believe not. For Jesus knew from the beginning who they were that believed not, and who should betray him.
65 And he said, Therefore said I unto you, that no man can come unto me, except it were given unto him of my Father.
66 From that time many of his disciples went back, and walked no more with him.
These disciples were simply those who accompanied Christ from the feeding of the five thousand along with the multitude. It is this multitude of unbelievers Christ is directly addressing while his disciples are observing this discussion rather than the audience being addressed by Christ.
Remember, by the time of his resurrection over "five hundred brethren" saw him at one time. Hence, although the twelve were selected as apostles there were many hundreds of professing disciples baptized under John the Baptist and His own twelve (Jn. 4:1-2) that followed him, and from among whom the replacement for Judas was selected (Acts 1:21-22).
The quotation from verse 44 in verse 64 had been addressed to the unbelieving crowd (see vv. 36-52) who needed to savingly believe in Jesus Christ as the bread of life, rather than to his disciples.
Indeed, one could not even be called a "disciple" in the gospel era unless they have confessed Christ and submitted to baptism already (Jn. 4:1-2; Lk. 7:29-30; Acts 19:5; Mt. 28:19-20).
In addition these are men he had known "from the beginning" but those he had addressed in the audience were those who had followed him from the miracle of the feeding of the thousands (Jn. 6:15-36).
Moreover, quoting again verse 44 and directly applying to those specific ones Christ claimed that were unbelievers from the beginning is set forth as an explanation why they remained in unbelief from the beginning as professing disciples:
64 But there are some of you that believe not. For Jesus knew from the beginning who they were that believed not, and who should betray him.
65 And he said, Therefore said I unto you, that no man can come unto me, except it were given unto him of my Father.
In addition, this text says again "no man can come unto me" which does not refer to calling the twelve as disciples but for coming to him for eternal life as that has been the consistent subject before (vv. 35-40) and after (vv. 46-51).
Finally, Peter explicitly proves the issue is not a calling to apostleship that is in view, but the "words of eternal life" that is being rejected or accepted (vv. 66-69).
I understand why your soteriological preference inclines you to this kind of interpretation, but I cannot see how the immediate context can support that kind of interpretation since it is coming to Christ for eternal life that is the consistent theme from John 6:29 to John 6:67-69.