John 6:28

They said therefore unto him, What must we do, that we may work the works of God? Jesus answered and said unto them, This is the work of God, that ye believe on him whom he hath sent.

Christ had just enjoined upon his hearers the mandate that they should work for the food that perishes not; and their reaction was quite naturally, "Well, what must we do?" thinking no doubt that he would mention some of the commandments from the Old Testament; but it was not merely a more particular fidelity to the Old Testament that could lead to eternal life, but the complete acceptance of an entirely new system that would be required not only of them but of all men. That new system of Christianity, though of grace and unmerited favor, was nevertheless a system with works of its own, works of a far different nature from the law, but still "works of faith," for Christ said in this verse: "Work for the food that abides unto eternal life" (John 6:27).

This is the work of God, that ye believe on him whom he hath sent ... In all the New Testament, there is not a more instructive verse than this which designates faith in the Lord Jesus Christ as a work performed by men, but also in the ultimate sense a work of God. This statement demolishes the prevalent Protestant heresy that "There is nothing you can do to be saved!" Faith itself is something that must be done; but its importance is sufficient to justify its standing here as a synecdoche of all that must be done. For more on synecdoche, see the index of my Commentary on Romans. As Dorris noted: "This verse illustrates the truth that the works of God are works ordained by God to be performed by men."[12] For identification of seven Scriptural classifications of works, see my Commentary on Romans, p. 62.

There is a difference in "believing" and in "believing in" or "believing on" the Lord Jesus Christ. For example, one believes the apostle John, but we do not believe in him. Christ demanded absolute faith in himself, and still does.

What must we do ... ? means "What must we do to be saved?" and is a question encountered several times in the New Testament. On Pentecost, in the jail at Philippi, and on the Damascus road, the question "What shall I do?" was the initial movement of souls toward the Lord. The question has a Scriptural answer, and it is criminal to substitute the sophistry of men for the divine answer. In answer to this question, the Holy Spirit said: "Believe on the Lord Jesus and thou shalt be saved ... Repent and be baptized every one of you in the name of Jesus Christ unto the remission of your sins ... Arise and be baptized and wash away your sins calling on his name" (Acts 16:31; 2:38; 22:16).