I think to get a full understanding one would do well to read 20 along with it.
Once we do that, it becomes clear that He is letting us know that man can not will a thing into being. Though as it talks about prophecy, he is making it clear that ma can not see what is coming, unless HaShem opens their eyes to it. Take John, when he wrote the Book of Rev. He could not have known any of what he seen, had not HaShem opened his eyes, and shown him.
Private interpretation.
They (the prophets) only speak what HaShem has gave them. They do not try to tell you what it means, unless they are told to. Also it is not open to ones own way of thinking, we must see it for what it is, and then it must fall into context with the whole of the Word, and not just the parts we hope fit.
A side note if you don't mind. One may also find in the passages, that the Spirit of HaShem, ( Ruach HaKodesh) is not something that just showed up after Yeshua, as some teach. It is and always will be that part of HaShem that speaks to us. Telling us when we do wrong, or when we are going to do something wrong.
Ever had the odd feeling that something just wasn't right? Then you find out latter that something had gone way wrong? That is what this is talking about. Well more or lass anyway.