The whole verse is "This is the work of God, that you believe in Him whom He has sent." Him == Christ
And this is the context
John 6:26–35 (NASB 2020) — 26 Jesus answered them and said, “Truly, truly, I say to you, you seek Me, not because you saw signs, but because you ate some of the loaves and were filled. 27 Do not work for the food that perishes, but for the food that lasts for eternal life, which the Son of Man will give you, for on Him the Father, God, has set His seal.” 28 Therefore they said to Him, “What are we to do, so that we may accomplish 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.” 30 So they said to Him, “What then are You doing as a sign, so that we may see, and believe You? What work are You performing? 31 Our fathers ate the manna in the wilderness; as it is written: ‘HE GAVE THEM BREAD OUT OF HEAVEN TO EAT.’ ” 32 Jesus then said to them, “Truly, truly, I say to you, it is not Moses who has given you the bread out of heaven, but it is My Father who gives you the true bread out of heaven. 33 For the bread of God is that which comes down out of heaven and gives life to the world.” 34 Then they said to Him, “Lord, always give us this bread.” 35 Jesus said to them, “I am the bread of life; the one who comes to Me will not be hungry, and the one who believes in Me will never be thirsty.
It is plain he is telling his audience they must believe-this is what God requires of them
and even a number of Calvinist sources affirm
John Calvin: “People who infer from this passage that faith is God’s gift are mistaken, for Christ does not show here what God produces in us, but what God wants and requires from us.” (The Crossway Classic Commentaries: John; Crossway Books; Wheaton, IL; 1994, p.393)
This is the work of God. This is the thing that will be acceptable to God, or which you are to do in order to be saved. Jesus did not tell them they had nothing to do, or that they were to sit down and wait, but that there was a work to perform, and that was a duty that was imperative. It was to believe on the Messiah. This is the work which sinners are to do; and doing this they will be saved, for Christ is the end of the law for righteousness to every one that believeth,
Ro. 10:4.11 Albert Barnes,
Notes on the New Testament: Luke & John (ed. Robert Frew; London: Blackie & Son, 1884–1885), 244.
Even the notes to the Geneva bible
Verse 29
5. Men torment themselves in vain when they try to please God without faith.
g. That is, this is the work that God requires, that you believe in me, and therefore he calls them back to faith.
Geneva Bible Notes (1599). (Bellingham, WA: Logos Bible Software, 2003), Jn 6:28–29.
Jesus sets them straight:
The work of God—
i.e. what God requires—is faith. This is not faith in the abstract, an existential trust without a coherent object. Rather, they must believe in the one [God] has sent
[1] Pillar New Tetament commentary D.A. Carson
John_6:28-29. Then said they unto him, What shall we do, that we might work the works of God? Jesus answered and said unto them, This is the work of God, —
The chief work, the greatest work which you can do. Spurgeon commentary
[1] D. A. Carson,
The Gospel according to John (The Pillar New Testament Commentary; Leicester, England; Grand Rapids, MI: Inter-Varsity Press; W.B. Eerdmans, 1991), 285.