spiritual at its root is the idea of things related to the spirit. spirit (in Scripture, at least) was the term for "air in motion." the implication is those things that are immaterial, invisible and yet powerful. the concept of scripture is that there are two worlds: the flesh (which is what we see, hear, feel) and that there is another world, the spiritual (immaterial, invisible and yet powerful). The god of Scripture is spirit, hence reigns in this spiritual world. religion (at its best, though often failing) is an attempt to combine those of flesh with the world of the spirit.