This is Nebuchadnezzar’s message to the empire about God’s personal dealings with him, which are about to be recounted in this chapter. It is written to glorify God.
Does this make him a believer, this strong statement in the one true God? We may hope so, but it is not necessarily the case. He has said things almost as strong as this before, and then put right alongside them approval of his own gods. He may be doing no more than counting the true God among the gods, and recognising him, but not as the exclusive God and the only God. As we go through the chapter we will find sentiments that can be interpreted in a polytheistic way. Nevertheless, as a public person he confesses his sin publicly, and he praises God and many think he was converted and delivered from polytheism.