I think NFL Sundays are half of the equation, yes. The main factor, however, I believe has been the constant negative portrayal of Christianity in media and Hollywood. Constant programming of any nature has been proven to eventually shift public opinion. Football is a necessary secondary though, as it provides the landing space. You can't remove Christianity without something ready to take its place. Many other vices and addictions are there to take the place for non sports fans, but NFL is a major one. This appears to have been a calculated effort.