Of course, he had told them, quite explicitly enough in the past, and even in his previous sermon in the temple, just two months earlier. They had questioned him before then, and through the chapters of John he has told them very plainly.
Before we can understand, before we are given the truth, before we will take it in, there must be a work in our hearts. We are so obdurate and stubborn. The Holy Spirit must illuminate us, and open our hearts, and give us, by his regenerating power, desire and need to hear, and readiness to believe. And then we are given a plain understanding. When anyone preaches the Gospel, or when you teach the Sunday school children and the youth, when you witness, we are dependent upon the Spirit of God to open eyes. Without that, people will understand nothing. We do not go into our acts of witness and preaching with self-confidence. Perish the idea that we should ever think: I am getting good at this, at explaining these things to people. We depend upon the prior work of God to open hearts. The cleverest people in the world will not really understand the question to believe it unless the Spirit prepares the heart. So we do everything we do with great dependence upon God and humility. Lord, open their hearts. Lord, enable them to see. We depend upon him. We pray to him.