I think you mean "foul revolt". Foul means disgusting, wicked or immoral. A revolt is a rebellion against a regime, government, authority, etc. So it means the disgusting betrayed that they made against God. It also wouldn't surprise me if - this being Milton - revolt was a carefully chosen word here, as it can also mean to feel disgust. There will be loads of resources online for Paradise lost. It's an extensively studied book. Do searches for (paradise lost notes / discussions / explanations / reading guides / etc). It's very hard work to read and is not usually studied until university level English in this country. I'm not saying don't read it but the concepts, parody, unexplained references and interpretation alone make it difficult. The old style of writing can be confusing to native speakers as well, so make sure you're very comfortable with your English before you take it on or I expect it will be very frustrating.

Join the club... I'm a fluent English speaker and it sounds very old and odd!