Summa contra Gentiles1.99 Chapter XCIX. That God's life is eternal

It follows from this that God’s life is eternal. For nothing ceases to live except by being severed from life. But nothing can be severed from itself: for all severance results from the separation of one thing from another. Therefore it is impossible that God cease to live, since Himself is His own life, as we have proved.?

Again. Whatever sometimes is and sometimes is not, results from a cause: for nothing brings itself from not-being to being: since what is not yet, acts not. Now the divine life has no cause, as neither thas the divine existence. Therefore He is not sometimes living and sometimes not living, but is ever living. Therefore His life is eternal.

Again. In every operation the operator remains, although sometimes the operation is transitory by way of succession: wherefore in movement the thing moved remains the same identically though not logically. Hence where action is the agent itself, it follows that nothing passes by in succession, but that the whole is throughout the same simultaneously. Now in God to understand and to live are God Himself, as was proved.? Therefore His life is without succession and is simultaneously whole. Therefore it is eternal.

Moreover. God is utterly unchangeable, as we have proved above. But that which begins or ceases to live, or is subject to succession in living, is changeable: for the life of a being begins by generation and ceases by corruption, and succession results from change of some kind. Therefore God neither began to be, nor will cease to be, nor is subject to succession in living. Therefore His life is eternal.;

Wherefore it is said (Deut. xxxii. 40) in the person of the Lord: I live for ever; and (1 Jo. v.).: This is the true God and life eternal.