John Chrysostom
He continually repeats the same thing in order to impress it upon the souls of His hearers, since this teaching was almost the last. He also wished to confirm faith in the dogma of the resurrection and eternal life.
Therefore Christ speaks of the resurrection partly because He has spoken of eternal life. He also speaks of it to show that this life is not now, but will be after the resurrection.
From where, you ask, is this clear? From Scripture. He sends the Jews to it on every occasion, commanding them to be assured from it.
By the words "gives life to the world" (Jn 6:33), He awakens zeal in them. In this way, at least from zeal that others enjoy the gift, they might not deprive themselves of it.
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He often mentions the manna in order to show the difference between it and the heavenly bread, and to lead them to faith.
If it was possible for Him, without harvest, bread, or other provisions, to preserve the life of their fathers for forty years, then much more will this be possible now, when He has come to accomplish greater things.
On the other hand, if those things were types, and yet then food was gathered without sweat and toil, much more will it be so now.
Now the difference is greater in everything. Death is no more, and we enjoy the true life.
It is good that Christ often mentions life, because life is desirable to men, and nothing is so pleasing as not dying.
In the Old Testament there was a promise of long life and many years. But now what is promised is not merely long life, but life that has no end.
At the same time Christ wants to show that He abolishes the punishment which followed from sin. He destroys that sentence of death and, contrary to the former decree, introduces not merely life, but eternal life.