Revealed religion is necessary to man only because he is so limited in his cognition. All the truths of revealed religion would be evident, or apparent, or demonstrable to us had we but knowledge of all the relevant items.
We can’t see the future, e.g.; so, much of John’s Apocalypse – and other apocalyptic – respects futurities. It’s not that we cannot know that there must eventually be an eschaton by way of natural theology, but that because, being temporal, we cannot know the details of the eschaton except by way of revelation, so that we cannot properly shape our doctrines accordingly. And some of those historical details of the eschaton are crucially important.
Or again, our lights cannot penetrate the veil of the Firmament, to see what transpires in other worlds – particularly the world of Heaven. Outside of revelation, then, we can’t know that there is war in Heaven. At most we can apprehend that something is Not Quite Right with the created order, and conjecture about the reasons. Once we learn from revelation that there is war, lots of other doctrines fall into place logically. In the light of revealed religion, we can see why truths discoverable by natural theology are true.
To a mind competent enough, revealed theology would be a department of natural theology.