What does the Bible teach about spirituality?

A person is spiritual when—and only when—he is filled by means of the Holy Spirit. Spirituality is an absolute—at any moment of time we are either one hundred percent filled by the Spirit and therefore spiritual, or we are not filled by the Spirit at all and are therefore carnal (1 Cor. 3:1–4; Gal. 5:16–17; 6:1). The indwelling of the Holy Spirit is a once-for-all thing that cannot be changed or lost, but with this indwelling comes an inner conflict between the Holy Spirit and our old sin nature, and our volition—our freedom to choose—is the arbiter in this conflict. We decide moment-by-moment who will be in control of our soul. The filling of the Spirit is a matter of choice, as we are commanded to be filled (Eph. 5:18), and we can receive or reject it by grace through faith. We lose the filling of the Spirit by choosing to grieve or quench the Spirit through sin or apathy, but we regain it by confessing our sins and being cleansed from all unrighteousness (1 John 1:9; Prov. 1:23). Being filled by the Spirit is not an end in itself, but rather the means to an end—the life that is produced by the consistent filling of the Holy Spirit is the life of Christ in us.