The christian religion is struggling in this modern world but I don't believe it is because we are too modern for it. Many people do not believe in the spiritualism of it but want to adhere to the philosophy. I think for many the idea that the bible has stories and ideas of hate and violence, genocide and chosen people of god is a real problem. The other problem for many is the hypocrisy of the leadership.
Phil Robertson, is an example of "look at these other people and their sins, not me or mine". One of the prominent traditions of the christian philosophy is to confess your sins and seek forgiveness and to repent. Another prominent teaching in the christian philosophy is not to judge. The bible is loaded with stories and warnings about judging others. Many christians will say they are not judging; that they are just stating what is sin. Context is everything.
In the old testament one of the oldest stories relating to not judging is the story of how the angels came to claim the body of Moses and when they did the devil was there and the story goes that they would not bring one railing accusation against him. According to the stories of the bible the devil put himself in the place of god. The bible repeats itself over and over again, god is the only one that can judge and to judge someone else is putting yourself in the place of god. The message of not judging is carried throughout the bible and unless you have no history of being taught the christian philosophy you are very aware of this teaching. In our modern world we have adopted this philosophy in a similar way. We do not allow law enforcement to judge the accused. We require a jury or at minimum a judge; evidence it presented and standards are set to find guilt.
The downfall of christianity I believe will eventually be traced back by historians to the judging of others based on the teachings of the philosophy. I have personally witnessed many people come to the religion and to accept the teachings and in doing so come to "know christ as their personal savior" and the greatest motivation for it that I witnessed was the forgiveness and the becoming part of something that they believe is sacred; That they will no longer be judged by their fellow man. The whole religion looses it's sacredness when the followers need to do nothing more than claim they believe and stand in judgement of others.
The fact that the religion is struggling to maintain followers is leading to desperation and the people who are leadership have basically adopted a system by which they will consider you part of the flock if you will just claim to believe in god or jesus and secondly you gain popular favor among the followers when you adopt popular bigotry against gays. The bible says; The lord will make the lowest or the least the highest and the highest the lowest. Welcome to a big gay world.