You have taken the John 6 Scripture partially out of its chapter context.
John 6:24-44
24 When the people therefore saw that Jesus was not there, neither His disciples, they also took shipping, and came to Capernaum, seeking for Jesus.
25 And when they had found Him on the other side of the sea, they said unto Him, "Rabbi, when camest Thou hither?"
26 Jesus answered them and said, "Verily, verily, I say unto you, Ye seek Me, not because ye saw the miracles, but because ye did eat of the loaves, and were filled.
27 Labour not for the meat which perisheth, but for that meat which endureth unto everlasting life, which the Son of man shall give unto you: for Him hath God the Father sealed."
The multitudes followed Jesus not because they believed on Him, but because He fed them. They loved the idea of free meals evidently.
28 Then said they unto Him, "What shall we do, that we might work the works of God?"
29 Jesus answered and said unto them, "This is the work of God, that ye believe on Him Whom He hath sent."
30 They said therefore unto Him, "What sign shewest Thou then, that we may see, and believe Thee? what dost Thou work?
31 Our fathers did eat manna in the desert; as it is written, 'He gave them bread from heaven to eat.'
They didn't see the miracle of His feeding them as anything more than what God did through Moses in the wilderness with the heavenly manna. So they wouldn't believe on Him just because of that working of the loaves and fishes.
32 Then Jesus said unto them, "Verily, verily, I say unto you, Moses gave you not that bread from heaven; but My Father giveth you the true bread from heaven.
33 For the bread of God is He Which cometh down from heaven, and giveth life unto the world.
Jesus speaking of Himself being sent from Heaven by The Father.
34 Then said they unto Him, "Lord, evermore give us this bread."
35 And Jesus said unto them, "I am the bread of life: he that cometh to Me shall never hunger; and he that believeth on Me shall never thirst.
36 But I said unto you, That ye also have seen Me, and believe not.
37 All that the Father giveth Me shall come to Me; and him that cometh to Me I will in no wise cast out.
Our Lord Jesus makes a distinction between those of them who believed not, and those whom The Father have given Him that will come to Him and believe.
38 For I came down from heaven, not to do Mine Own will, but the will of Him That sent Me.
39 And this is the Father's will which hath sent Me, that of all which He hath given Me I should lose nothing, but should raise it up again at the last day.
40 And this is the will of Him That sent Me, 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.
Again, He mentions the condition of "that of all which He hath given Me" from The Father.
41 The Jews then murmured at Him, because He said, I am the bread which came down from heaven.
42 And they said, "Is not this Jesus, the son of Joseph, whose father and mother we know? how is it then that He saith, I came down from heaven?"
43 Jesus therefore answered and said unto them, "Murmur not among yourselves."
44 No man can come to Me, except the Father Which hath sent Me draw him: and I will raise him up at the last day.
KJV
Again Jesus makes the distinction of only those whom The Father has drawn can come to Jesus. It is why Christ's Apostles asked Him in Matthew 13 why He spoke to the multitudes in parables, and He said it was not given to them to know the mysteries of the kingdom of heaven.
Thus the John 6 Message is not a Message that all we need do is believe on Jesus.
There is no Message there in John 6 about 'how' we are to serve Christ Jesus after having believed on Him. There is plenty Scripture written later by Apostle Paul on 'how' we are to serve Jesus after having believed on Him and been baptized.
That John 6 Chapter is about initial first belief. Those multitudes who followed Him didn't believe on Him, but believed on the free food handout. And then some of the Jews among them began murmuring because what He said about coming down from heaven was close to saying He had some special relationship with God, while they only saw Him as the carpenter's son. At the end of John 8 is really where Jesus reveals to the Jews His claim to be The I AM (God's sacred name).