Doctorow addresses the the issue of God by intertwining a series of narratives, reflections, and commentaries. The tie that holds all these narrative threads together is the story of the theft of a cross from an Episcopal church and its subsequent discovery on the roof of a synagogue. This theft and investigation bring together the iconoclastic priest of the episcopal church and the female rabbi at the synagogue.
Doctorow's theme is the necessity to strip away all the trappings and build a belief system based on the most primal yearning for God. For Doctorow that means religion and God are what brings us closer to each other in either love or a quest for justice.
City of God is thought provoking and so well written that it draws you in as you read it. However, Doctorow's method of jumping from thread to threat is distracting. For me, I was almost a third of the why through the book before I was able to untangle the threads to get a handle on what is going on. As a card carrying(and paycheck receiving) member of the professionally religious, I have my doubts about being able to strip away all the trappings to get to the essence of religion.
Doctorow focuses on Christianity and Judaism and the essence of those religions is that God communicates through the events of history. In effect the method and the message are indistinguishable from each other. To strip away the method is to strip away the message, and Doctorow seems to want to do both; even though his different narratives make underlie the idea of the method is the message.
Even though I had some quibbles with City of God, I am impressed enough by Doctorow that I want to read some more of his books.