John 6:27-29
27 "Do not work for the food which perishes, but for the food which endures to eternal life, which the Son of Man will give to you, for on Him the Father, God, has set His seal."
28 Therefore they said to Him, "What shall we do, so that we may work the works of God?"
29 Jesus answered and said to them, "This is the work of God, that you believe in Him whom He has sent."
NASU
Faith is work, our Lord said so. The people asked what shall we do! Jesus had a great opportunity to correct them, but He did not say to them " You cannot do any work", He told them to believe in Him, this is the work of God!
Christ responded: “This is the work of God (i.e., from God – a genitive of source), that you believe on him whom he has sent” (6:29).
It is not God who believes for you, yet He is the source in which we can believe, but it is the people who must put their trust in Christ.
When the Philippian Jailor asked "WHAT MUST WE DO?" Maybe Paul should have told him: "There is nothing you can do" and then maybe Jesus should have said the same thing.