In my experience those with faith are a lot more content with life than atheists, purely because they have hope - hope is a powerful thing. Equally they always have someone to support and guide them in times of need - God. Whether he exists or not isn't important, but the effect that the belief in him has on people is incredible.
I would argue that the nihilistic view that the world and life is ultimately pointless, and we should live for today and overindulge in pointless hedonism, along with the fear of death and the pressure to live our lives to the fullest, is far more damaging.
It's the simpler things in life that are the root of contentment & happiness - love, family , community, goodness, spirituality - All things that Christianity actively encourages, and society is losing.
But you are mistaking faith with religion, I have faith but I do not give a toss for religion or God as long as each of us have faith in something it is enough for me my family and friends provide me with faith and I am happy with life. To claim that non believers cannot be completely happy with life is a typical religious myopic view and why most atheists have no time for religion in any shape or creed.
Christianity was a human creation to control the masses no more.