To do less dooms it to become a mere shell. Much like a cadaver. It still looks human, but it no longer acts human. And like a cadaver, faith can remain a mere shell only for so long. Then the forces of decomposition reduce it to a skeleton, then to dust, then to memory.
Faith works out.
Exercise develops strength. Exercise manifests strength. Exercise tests strength. Exercise invigorates strength. Take the answer to that question and apply them to the exercise of faith, which in this passage is likened to a spiritual body.