John 6:27

“Do not work for food that perishes, but for food which continues unto eternal life which the Son of Man will give you, for him the Father, even God, has sealed.”

So He emphasises that they are not to put their efforts into obtaining food that can only go bad, but into obtaining the spiritual food which never goes rotten but goes on and on feeding the soul and resulting in eternal life (compare Matthew 6:19; 2 Corinthians 4:18; Colossians 3:1). This is the kind of food which the Son of Man has come to give them, and it is on Him that God the Father ‘has set His seal' (John 6:26). Note the references to the Son of Man, and to the fact that God was His Father and had sealed Him. The whole atmosphere is Messianic, and more.

So He knew that they were not there because they had recognised from His signs Who He really was and what He had come to do, nor because they were seeking spiritual life, but merely because they wanted a leader who would constantly be able to supply them with their worldly needs. The miraculous feeding had been intended to show them that God could also feed their souls, and possibly also to stress that by coming to Him they were becoming part of the covenant community, but they were merely taking it to mean that He could look after their bodily cravings.

Thus He stressed that they must put their efforts into finding soul food, ‘the food which endures to eternal life', food that would go on benefiting them for ever and give new life, the life of the new age. And He, as the Son of Man, Who was licensed by God the Father to give this food, is the One to Whom they should look for it.

‘Has set his seal.' The setting of the seal provided the recipient with the right to act in the stead of the sealer, as His representative. That seal was set for Jesus at His baptism when the voice spoke from Heaven. It was confirmed by His miracles and the feeding of the great crowd and by all that He did.

‘The Son of Man.' John only brings out this title when he has something very significant to say. So in John 1:51 the Son of Man was the one on whom the angels ascended and descended, indicating that God was with Him in the fulfilling of some special divine purpose. In John 3:13 He is the One Who has come down from Heaven and must be lifted up. In John 5:27 He is the One authorised to carry out judgment. In John 6:53 He is the One without Whose death there can be no eternal life. In John 6:62 He is the One Who will rise to where He was before. In John 8:28 He is to be lifted up. In John 12:23; John 13:31 He is destined to be glorified. Thus in John the title ‘Son of Man' is far from simply indicating His humanity, or even Messiahship (although it does include that). Note its paralleling with the idea of ‘the Son' in John 6:40; John 6:53. It is rather concentrating on the idea of the Son of Man as a figure connected with Heaven Who has come to earth with a divine purpose to fulfil, a purpose linked to His death, and which will result in final triumph. Here in particular He is seen as the giver of eternal life.