This doctrine, although it doesn't appear by name in the Bible, is the central tenet of Christianity. It states that Jesus Christ, the Second Person of the Trinity (see Arius), "took on" flesh (in carne, "in flesh"), or became a man. In other words, God became human, was born as a baby, and was later killed by humankind in order to become the substitute sacrifice for the propitiation of sin (see Christianity). This does not mean that God ceased to exist other than within the flesh of Jesus of Nazareth. It means that the eternal "Word" of God became the "Son" of God, present now in time. This concept is called the hypostatic union—perfect God and perfect man in one body. "The Word became flesh and dwelt among us" (John 1). Whenever the "other" steps across the line separating material from spiritual and becomes human, or "takes on" humanity, the divine is said to be "incarnate," or "in the flesh."
The term is used in a slightly different form outside of Christianity. Hindu belief, for instance, sees the human spirit (Atman) existing, over the course of many lifetimes, in many different bodies. When the spirit or soul incarnate in flesh takes on a new form, a new body, it is said to be reincarnate, or incarnated again. This is the doctrine of reincarnation.