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.
Related Commentaries of John 6:36
John 6:36
36 But I said unto you, That ye also have seen me, and believe not.