The following is an excerpt from The MacArthur New Testament Commentary on John 6.

Jesus answered them and said, “Truly, truly, I say to you, you seek Me, not because you saw signs, but because you ate of the loaves and were filled. 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.” Therefore they said to Him, “What shall we do, so that we may work the works of God?” Jesus answered and said to them, “This is the work of God, that you believe in Him whom He has sent.” (John 6:25–29)

As it does throughout John’s gospel, the solemn affirmation amen, amen (truly, truly) introduces an important truth to which Jesus wanted His hearers to pay careful attention. The Lord’s rebuke, “you seek Me, not because you saw signs, but because you ate of the loaves and were filled,” laid bare their selfish, materialistic hearts. So blinded were they by their superficial desire for food and miracles that they missed the true spiritual significance of Jesus’ person and mission. “They were moved not by full hearts, but by full bellies” (Leon Morris, The Gospel According to John, The New International Commentary on the New Testament [Grand Rapids: Eerdmans, 1979], 358). Though they had witnessed the miraculous signs Jesus had performed (v. 14), they failed to grasp the spiritual implications of those miracles.

Jesus rebuked the crowd for their crass materialism. Instead of working for the food which perishes, the physical food they sought, Jesus exhorted them to pursue the food which endures to eternal life (Jesus Himself, the Bread of Life; vv. 35, 54). While He was certainly aware of their need for physical nourishment (cf. vv. 10–12), He was much more interested in their spiritual well-being. As He earlier had distinguished physical water from the “water springing up to eternal life” (4:14), Jesus here pointed His hearers away from literal food to Himself as the Bread of Life (vv. 33, 35, 48, 51). Rather than focusing on the decaying outer man (2 Cor. 4:16), they needed to seek the spiritual nourishment which only the Son of Man can give. After all, to gain the whole material world but forfeit one’s eternal soul profits nothing (Matt. 16:26; Luke 12:16–21). As the one on whom the Father, God, has set His seal of approval, Jesus has the authority to dispense the spiritual food that comes from God and satisfies the hunger for righteousness (Matt. 5:6).

In response to Jesus’ command in verse 27 to pursue the spiritual, nonperishing food of eternal life, the people said to Him, “What shall we do, so that we may work the works of God?” They filtered Jesus’ words through their own warped minds and thought He was saying they needed to do some works to earn eternal life. What was that work they should do, they wondered. Similarly, the rich young ruler asked Him, “Teacher, what good thing shall I do that I may obtain eternal life?” (Matt. 19:16), and in Luke 10:25 “a lawyer stood up and put Him to the test, saying, ‘Teacher, what shall I do to inherit eternal life?’ ” It was a familiar matter for the Jews to pursue eternal life through their religion, so the question was common.

True salvation, of course, is not by works (Titus 3:5). Thus, Jesus answered their question by noting that the only work acceptable to God is to believe in Him whom He has sent. Salvation is by grace alone (Eph. 2:8–9) through faith alone (Rom 3:28) in Christ alone (Acts 4:12), “because by the works of the Law no flesh will be justified in His sight” (Rom. 3:20; Gal. 2:16). Salvation is the gift of God (John 4:10; Rom. 5:15; 6:23; Eph. 2:8).

Thus, salvation does not come from human effort, achievement, or moral works, but from a faith that inevitably produces good works (Eph. 2:10; cf. Matt. 3:10; 7:16–20; 12:33; 13:23; Luke 6:43–46; Eph. 5:8–9; Col. 1:10). A “faith” that does not produce fruit is dead, meaning that it is not really biblical faith at all (James 2:14–26).

The rest of John chapter 6 develops Jesus’ teaching regarding the food which endures to eternal life. Pointing to Himself as the Bread of Life, Jesus offered Himself to His listeners as their eternal deliverer. The crowd, however, was ultimately not interested. They had been intrigued by His earlier healings, and they had been temporarily satisfied by His miraculous meal. But their initially enthusiastic response to those supernatural signs (v. 15) quickly faded when their superficial expectations went unmet. Unlike the Twelve, who responded to Jesus’ power with praise, the crowd responded first with curiosity, but unwilling to abandon their false righteousness and repent, they were left finally with rejection. Although they followed Christ for a little while, even sailing across the Sea of Galilee to find Him, they eventually demonstrated that they were not true followers at all.