Chapter 14 — Part I: The Creed — Article XII: Life Everlasting
Section 7. These words, life everlasting, also teach us that, contrary to the false notions of some, happiness once attained can never be lost. Happiness is an accumulation of all good without admixture of evil, which, as it fills up the measure of man's desires, must be eternal. He who is blessed with happiness must earnestly desire the continued enjoyment of those goods which he has obtained. Hence, unless its possession be permanent and certain, he is necessarily a prey to the most tormenting apprehension.