One reason is articulated brilliantly by Tim Keller:
The real secret of fruitful and effective mission in the world is the quality of our community. Exceptional character in individuals cannot prove the reality of Christianity. Atheism, as well as many other religions, can also produce individual heroes of unusual moral greatness. Though such individuals may inspire us, it is all too easy to conclude that these individuals are just that - extraordinary heroes who have set unattainable standards for the rest of us. What atheism and other religions cannot produce is the kind of loving community that the gospel produces. In fact, Jesus states that our deep unity is the way the world will know that the Father sent him and has loved us even as the Father has loved him (John 17:23). Jesus says that the main way people will believe that Christians have found the love of God is by seeing the quality of their life together in community.
Timothy Keller, Center Church, p.311.
