I agree that Gray's criticism of secular humanism is arbitrary. He stops his historical analysis short, but once we realize that Christianity borrowed its ideas from other sources, and indeed that all cultures are based on other cultures, the whole point of reducing a cultural product to its source is lost. Which source? How far back do you want to go? Why bother committing the genetic fallacy in that manner anyway? Why not be charitable and take the cultural product at face value, to assess its present utility?