The relationship between humanism and organized religion (especially Christianity) can best be described as the marriage of a high school football jock to his attractive yet brain dead cheerleader girlfriend; first it’s all roses, then he has some second thoughts, and finally, thirty years later, they end up divorced and frequently sending each other letter bombs. St. Thomas More, one of the first, and certainly one of the most influential renaissance humanists, writer of the book Utopia, was also catholic. McKay goes into great detail when it comes to renaissance humanists, with one popular variety being christian humanists. Clearly, during the renaissance, religion and humanism were on the best of terms.
Fast forward to the present, and the situation is rather different. The America Humanist Association has some rather strong opinions about the bible, and most other humanists seem to agree with those sentiments. One of the most well known humanists of modern times was Christoper Hitchens, who passed away last December. To say that Mr. Hitchens was not a fan of religion would be a massive understatement. Christopher Hitchens was a strong opponent of religion both as it applied to politics, and to normal life. He railed against it in various newspaper and magazine columns (he wrote book review for the New York times among other things), and even wrote a book on the subject, titled God Is Not Great: How Religion Poisons Everything. As I said, he was not what you would call a fan of religion.
So the question is, why did humanism change this way? In my opinion, the change was inevitable. A large part of humanism revolved around the idea that people can make life better for themselves right here on this earth rather than relying on the afterlife. As technology advanced, and as the human standard of living continued to improve, humanists probably increasingly considered the traditional ideas of western religion to be an impediment to the advancement of the human condition through science, as well as a catalyst for illogical decision making. Whatever the reason behind the shift in the humanist attitude towards religion, the subject definitely warrants closer scrutiny.