That sentence is so convoluted it seems like a joke. Aha. Thank you, Google. It's from George Orwell's "Politics and the English Language." He is quoting Harold Laski, and he is quoting it as an example of very bad English style. Later on he pokes fun at double-negative constructions like "not unlike." He suggests that if you are doing it yourself, you should try to cure yourself by saying "A not unblack dog was chasing a not unsmall rabbit across a not ungreen field." So, what does it mean? Well, leave out Shelley, that's an aside. It has something to do with this question: "Did Milton become, out of an experience ever more bitter in each year to the founder of that Jesuit sect?" "I am not, indeed, sure whether it is not true to say" means "I think it IS true to say." I think Laski means: "I think that Milton, who once seemed like a seventeen-century version of Shelley, became more and more bitter toward the founder of the Jesuit sect..."

The main point is this : the writer is not sure if Milton disliked the founder of the Jesuit sect more and more every year. It is a hard sentence to make sense of, even for a native English-speaker.