In the Christian faith, what constitutes being alive as a human is possessing a soul and the associated ability to reason and use language. In the hierarchy of being, the mineral kingdom possesses durability in greatest measure, the plant kingdom the ability to take in nourishment, the animal kingdom, sensation or perception, the human kingdom reason, and the angelic one adoration. Thus  humans are defined as alive qua humans when they have souls. In the history of Christianity, this was originally defined as occurring after birth -- in some cases, in fact, after language acquisition -- but over time different Christian denominations have defined it as occurring at various points in the gestation process, and some even earlier. With changes in medical technology, including the ability to sustain life in a vegetative state, religions need to define more precisely the point at which the soul departs the body; because such end-life technology did not exist in early Christianity, there really are not good parallels or precedents.