(CG 1/098). That God is His own Life (Book 1: Of God As He Is In Himself) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

IT is impossible for God to cease to live, since Himself He is His own life (Chap. XCVIII).

2. Everything that at one time is and at another time is not, has existence through some cause. But the divine life has no cause, as neither has the divine being. God is therefore not at one time living and at another not living, but always lives.

3. In every activity the agent remains, although sometimes the activity passes in succession: hence in motion the moving body remains the same in subject throughout the whole course of the motion, although not the same in our consideration. Where then the action is the agent himself, nothing there can pass in succession, but all must be together at once. But God's act of understanding and living is God Himself (Chapp. XLV, XCVIII): therefore His life has no succession, but is all together at once, and everlasting.

Hence it is said: This is the true God and life everlasting (i John V, 20).