The approach of the modern world was very threatening to religion and the concept of God. Things which had previously been accepted on faith or because the Bible or the Church told us it was so were increasingly coming into question.
It is in this environment which Leibniz, Voltaire's model for Dr. Pangloss in Candide and Spinoza, the moral atheist, formulated their differing, yet intertwined, philosophies.
Stewart's argument is that Leibniz and Spinoza were both ahead of their time in understanding the portent of modernity but that they reacted to it very differently. Spinoza welcomed and embraced the shifting definition of God while Leibniz did all that he could to forestall the impending storm.
As a philosophical dilettante, I found this book hard and yet fascinating. Many a paragraph I had to read and reread, only to still not quite grasp its point. For all that, Stewart presents the conflict between the two men in an engrossing way which kept me reading through the depth.
I came away finding references to these two philosophers recurring around me and am inspired to read more. I've already started to reread Candide which is, in part, Voltaire's critique of Leibniz' theories. I'm also intrigued by Richard Dawkin's The God Delusion which draws on Spinoza and which was featured with an interview of Dawkins on Fresh Air the same night I finished this tome.