John 6:29~
29 Jesus answered them, “This is God’s work: to believe in the one whom he has sent.”

This text is used in a number of ways that abuse it and really miss the point of what Jesus is saying here. It is not saying that faith is this meritorious work that saves. Nor is it the point that our faith is the product of Divine activity. Even though that is true, I think it misses what this text is saying. We force issues into this text and force it into the Works vs faith debate. We need to read it in the context of Jesus commanding these people to believe in him.

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. Such language may reflect a specific Old Testament passage, such as Malachi 3:1 where God promises to send, in due time, the ‘messenger of the covenant’, but in fact the language is reminiscent of the entire ‘sentness’ theme in the Fourth Gospel. Jesus is supremely the one who reveals God to us, precisely because, unlike any other person, he has been in the courts of heaven and has been sent from there so that the world might be saved through him (e.g. 3:11–17). Faith, faith with proper Christological object, is what God requires, not ‘works’ in any modern sense of the term. And even the faith that we must exercise is the fruit of God’s activity (cf. notes on vv. 44, 65). Although the noun ‘faith’ is not used, this ‘work of God’ turns out to be nothing else than faith, making this ‘work of God’ diametrically opposed to what Paul means by ‘the works of the law’. As a result, the thought of the passage is almost indistinguishable from Paul: ‘For we maintain that a man is justified by faith apart from observing the law’ (Rom. 3:28).

Carson, D. A. (1991). The Gospel according to John (p. 285). Leicester, England; Grand Rapids, MI: Inter-Varsity Press; W.B. Eerdmans.

29 Jesus replaces their “works of God” with the singular “work of God.” But one thing is needful. And this one thing, he makes plain, is faith.81 They must believe82 on him (for the construction see on 1:12; the present tense here denotes the continuing attitude, not the once-for-all decision). In view of the controversy over faith and works reflected in the Epistle of James, it is interesting to find Jesus describing “work” as believing: God does not require that we pile up merits to obtain a heavenly credit. He requires that we trust him. The “work of God” means that which God requires of us. “There is a sense in which ‘to believe’ is to perform a work” (MacGregor). The object of faith is spoken of in terms of mission. Jesus is sent by the Father (“he” is emphatic83 — none less than he!), and this makes faith in him eminently reasonable.

Morris, Leon. The Gospel According to John (The New International Commentary on the New Testament) (Kindle Locations 8863-8870). Eerdmans Publishing Co – A. Kindle Edition.

29 The people, however, do not respond to this clear reference to the Son of man, perhaps because they were not familiar with this figure (cf. 9:35, 36; 12:34). But they do understand that Jesus is urging them to seek “something higher” than what has up to this point motivated them to follow him. They therefore ask him what they must do (picking up the cue from Jesus’ words) “to be doing the works of God.”107 Jesus’ answer (vs. 29) tersely expresses the issue at stake here as in all the conversations and encounters of the Fourth Gospel: “This is the work of God, that you believe in him whom he has sent.” The idea is not to elucidate the concepts “works” and “faith” in their mutual relations, as in Paul. All that is at stake here — which also governed the preceding — is that Jesus, including by means of the preceding miracle, pursues but one goal with the people: to open their eyes to the fact that in his coming and work God is in the process of addressing himself to them with his redeeming action, of introducing his kingdom, and of fulfilling his promise. In that connection the continual reference to the Son of man serves, over against all earthly and temporal patterns of expectation, to maintain the divine character and content of his messianic mission. For it is the person of the sent one and the way to which God will hold him that imparts to the food he gives its imperishable character (cf. vss. 5 Iff.). And the appropriate attitude on the part of those for whom this is intended is openness for what God gives and does, not works as a human achievement. That this “believing in him” is nevertheless called “the work of God”108 has its roots in the pronouncement in vs. 27. All the labor and effort the people are investing to remain near Jesus is vain and unprofitable — a dead work — as long as they follow him on the basis of human expectations and not on the basis of faith.

Ridderbos, Herman. The Gospel of John: A Theological Commentary (Kindle Locations 6581-6597). Eerdmans Publishing Co – A. Kindle Edition.