It is even possible that this saying was already recognisable to the followers of Jesus from its use at the Lord's Supper before they read it in John's gospel. There is a good deal for us to learn from this. From the point of view of the evangelist telling the story of Jesus and his forerunner John the Baptist, a life of faith need not be lived without questions. Faith is not blind belief, but open to evidence. John was able to ask, and Jesus gave him an answer in terms of looking and seeing.
Faith is about seeing the whole of it, knowing what the evidence means.
We too are invited to look and see and learn from the evidence. Indeed that is why the gospels were written. Look at the evidence, see for yourself. But what he pointed to, and the words Jesus used to describe it were not mere novelties, or even straight forward observations, which people might each interpret in their own way. He pointed to signs that John would have known from scripture.