John 6:26-29 — Jesus answered them and said, "Verily, verily, I say unto you, Ye seek Me, not because ye saw the miracles, but because ye did eat of the loaves, and were filled.
Labor not for the meat which perisheth, but for that meat which endureth unto everlasting life, which the Son of man shall give unto you: for Him hath God the Father sealed."
Then said they unto Him, "What shall we do, that we might 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."
The crowd looked for Jesus, not to believe on Him, or even to see His miracles, but because He gave them food — a selfish, carnal reason.
They had worked hard chasing Him for perishable food. He tells them to look for unperishable food — eternal life — that comes without work.
God the Father sealed — Jesus was accredited by God, sealed by the Spirit and empowered to give eternal life.
What shall we do? — even though Jesus had just said He would give it to them.
that we might work — They sought to satisfy God with their own efforts, to make themselves acceptable to God.
Jesus said, "believe on Me." Now to him that worketh is the reward not reckoned of grace, but of debt. But to him that worketh not, but believeth on Him that justifieth the ungodly, his faith is counted for righteousness (Romans 4:4-5).
Jesus offers food that will last for eternity and then later reveals that He is that food.
seal = sign of ownership and authenticity.