The reason Darwin had to mention God in his book, was the mindset of that time. Like so many have alread admitted, and understand, Creation was the accepted thing at that time. And the biggest precent of the population were Christian. So to attack this, in the form that Darwin could have done, would have been sure death for his book, and his idea. Because every pulpit would have been preaching against it, and people would of had preconcieved ideas, even before reading the book. So God was mentioned because Darwin was not stupid to not do so.