George Eliot stated that this book had been written with "my blood." It is a mass of detail, so much so that the detail often obscures the story, and must have involved hours of study - and no Internet in those days, either! To my mind the end result is not very pleasing. Admittedly there are some excellent passages in the book, but the overall impression is not one of delight, and I don't have the desire to read it again; rather, a sense of satisfaction that the end has been reached.
The story is set in Florence in the years 1492 -1505: the years in which Savonarola came to exert a powerful influence over the multitude of the people. He is the central figure in the novel in that he influences Romola, and the action she takes following her contact with him, determines how the story progresses.
It is quite obvious that Eliot deals with some of the problems of faith that troubled her during her break with Evangelicalism. She chooses Savonarola as the means of being able to ruminate upon the difficulties of faith such as miracles, power, and conscience. She also uses Romola who is converted to the faith and then, later on, is caused to question it. Eliot also question her faith.
Much of what Eliot says about religion rings true today. For example she comments on the attitude of the young people of Florence who, under the influence of Savonarola, have been walking the streets in groups seeking to pressurise people into giving up their trinkets etc., "To coerce people by shame, or other spiritual pelting, into the giving up of things it will probably vex them to part with, is a form of piety to which the boyish mind is most readily converted; and if some obstinately wicked men get enraged and threatened the whip or cudgel, this also was exciting." This is one of the truths about human nature: it loves to stop other people from doing the things it once enjoyed. Eliot comments on this twisted understanding of the Christian life when she reveals Romola supporting Savonarola in an effort to prevent the people reading books that are about evil. Romola confesses she has read them. Her companion replies, "Yes, yes, it's very well to say so now you've read them."
Finally, then, the book deals with many of the problems of faith that arise in the thinking person's mind in an attempt to reconcile faith and life itself. Not all are fully answered but Eliot tries to answer - to some satisfaction.