But I said unto you - This he said, not in so many words, but in substance, in John 6:26. Though they saw him, and had full proof of his divine mission, yet they did not believe. Jesus then proceeds to state that, although they did not believe on him, yet his work would not be in vain, for others would come to him and be saved.

But I said unto you,.... The substance of what follows in John 6:26 though the Persic and Ethiopic versions render it, "I say unto you"; and so refers not to anything before said, but to what he was about to say:

that ye also have seen me, and believe not; that is, they had not only seen him in person, which many kings, prophets, and righteous men had desired, but not enjoyed, yet nevertheless believed; but they had seen his miracles, and had shared in the advantages of them, being healed, and fed corporeally by him, and yet believed not in him as the spiritual Saviour and Redeemer of their souls; nor did they come to him in a spiritual way, for eternal life and salvation.

But

Though you have seen as you asked, I repeat what I said to you that you have seen and do not believe.

But I said unto you, That ye also have seen me, and believe not.

6:36 Ye also have seen me, and believe not. They had asked a work in order that they might believe, which was a confession of their unbelief. They ask for the bread of life, but they can only partake of it by believing in him. He therefore points out the one obstacle to obtaining what they had asked for.

6:36 I have told you - Namely, John 6:26.

36. But . ye have seen me, and believe not-seen Him not in His mere bodily presence, but in all the majesty of His life, His teaching, His works.

6:36-46 The discovery of their guilt, danger, and remedy, by the teaching of the Holy Spirit, makes men willing and glad to come, and to give up every thing which hinders applying to him for salvation. The Father's will is, that not one of those who were given to the Son, should be rejected or lost by him. No one will come, till Divine grace has subdued, and in part changed his heart; therefore no one who comes will ever be cast out. The gospel finds none willing to be saved in the humbling, holy manner, made known therein; but God draws with his word and the Holy Ghost; and man's duty is to hear and learn; that is to say, to receive the grace offered, and consent to the promise. None had seen the Father but his beloved Son; and the Jews must expect to be taught by his inward power upon their minds, and by his word, and the ministers whom he sent among them.