12 “They said therefore unto Him, What shall we do, that we may work the works of God?” For He had said to them, “Labor not for the meat which perishes, but for that which endures unto eternal life.” “What shall we do?” they ask; by observing what, shall we be able to fulfill this precept? “Jesus answered and said unto them, This is the work of God, that you believe in Him whom He has sent.” This is then to eat the meat, not that which perishes, but that which endures unto eternal life.
To what purpose do you make ready teeth and stomach? Believe, and you have eaten already. Faith is indeed distinguished from works, even as the apostle says, “that a man is justified by faith without the works of the law:” there are works which appear good, without faith in Christ; but they are not good, because they are not referred to that end in which works are good; “for Christ is the end of the law for righteousness to every one that believes.” For that reason, He wills not to distinguish faith from work, but declared faith itself to be work.
For it is that same faith that works by love. Nor did He say, This is your work; but, “This is the work of God, that you believe in Him whom He has sent;” so that he who glories, may glory in the Lord. And because He invited them to faith, they, on the other hand, were still asking for signs by which they might believe. See if the Jews do not ask for signs. “They said therefore unto Him, What sign will you do, that we may see and believe you? What do you work?” Was it a trifle that they were fed with five loaves?
They knew this indeed, but they preferred manna from heaven to this food. But the Lord Jesus declared Himself to be such an one, that He was superior to Moses. For Moses dared not say of himself that ge gave, “not the meat which perishes, but that which endures to eternal life.” Jesus promised something greater than Moses gave. By Moses indeed was promised a kingdom, and a land flowing with milk and honey, temporal peace, abundance of children, health of body, and all other things, temporal goods indeed, yet in figure spiritual; because in the Old Testament they were promised to the old man.
They considered therefore the things promised by Moses, and they considered the things promised by Christ. The former promised a full belly on the earth, but of the meat which perishes; the latter promised, “not the meat which perishes, but that which endures unto eternal life.” They gave attention to Him that promised the more, but just as if they did not yet see Him do greater things. They considered therefore what sort of works Moses had done, and they wished yet some greater works to be done by Him who promised them such great things.
What, say they, will you do, that we may believe you? And that you may know that they compared those former miracles with this and so judged these miracles which Jesus did as being less; “Our fathers,” say they, “did eat manna in the wilderness.” But what is manna? Perhaps ye despise it. “As it is written, He gave them manna to eat.” By Moses our fathers received bread from heaven, and Moses did not say to them, “Labor for the meat which perishes not.” You promise “meat which perishes not, but which endures to eternal life;” and yet you work not such works as Moses did. He gave, not barley loaves, but manna from heaven.
Source: Tractates on the Gospel of John (New Advent)