"Just as each one of us has one body with many members, and these members do not all have the same function, so in Christ we who are many form one body, and each member belongs to all the others."
--Romans 12:4-5
This is merely a statement of Biology at the core. Had a body not all of it's individual parts it would cease to be a body and be simply a collection of eyes, teeth or intestines.
Now, taking this analogy into the realm of Religion in general. Every religion serves a purpose. Every religion a function in the body of life. We learn of Compassion and Self Enlightenment from Buddhism. We learn are balance in life from Toaism. We learn of God's presence in us from Paganism. We learn the best laws to operate soceity from the Jews. We learn from Christianity the human need for forgiveness.
Why Atheism though? How is this necessary for religion in our world?
With all the religions in the world it is easy to accept anything that sounds "God"ish or "religion"ish. Then we are led astray into a religion of myths and fables. The role of Atheism in our world, a naturally religious one, is to purify beliefs. We learn from Atheism to examine everything thoroughly before giving it credence.
We learn not to accept everything at face value just because we are told to do so, but to examine it for cause and reason.
Atheism is like the Auto-Immune System that rejects beliefs that don't belong.