(CG 3/61). That by the Sight of God one is Partaker of Life Everlasting (Book 3. God, the End of Creatures) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

WHATEVER now is, and now is not, is measured by time. But the vision that makes the happiness of intellectual creatures is not in time, but in eternity (Chap. LXI). It is impossible therefore that from the moment one becomes partaker of it he should ever lose it.

2. An intelligent creature does not arrive at its last end except when its natural desire is set at rest. But as it naturally desires happiness, so it naturally desires perpetuity of happiness: for, being perpetual in its substance, whatever thing it desires for the thing's own sake, and not for the sake of something else, it desires as a thing to be had for ever. Happiness therefore would not be the last end, if it did not endure perpetually.

3. Everything that is loved in the having of it brings sadness, if we know that at some time we must part with it. But the beatific vision, being of all things most delightful and most desired, is of all things most loved by them who have it. They could not therefore be otherwise than saddened, if they knew that at some time they were to lose it. But if it were not meant to last for ever, they would be aware of the fact: for in seeing the divine substance, they also see other things that naturally are (Chap. LIX).

6. It is impossible for one to wish to resign a good thing that he enjoys, except for some evil that he discerns in the enjoyment of that good, or because he reckons it a hindrance to greater good. But in the enjoyment of the beatific vision there can be no evil, since it is the best thing to which an intelligent creature can attain: nor can he who enjoys that vision possibly think that there is any evil in it, or anything better than it, since the vision of that sovereign truth excludes all false judgement.

5. Nothing that is viewed with wonder can grow tedious: as long as it is an object of wonder, the desire of seeing it remains. But the divine substance is always viewed with wonder by any created intelligence, since no created intelligence can comprehend it. Therefore such intelligence can never find that vision tedious.

9. The nearer a thing comes to God, who is wholly unchangeable, the less changeable it is and the more enduring. But no creature can draw nearer to God than that which beholds His substance. The intelligent creature then gains in the vision of God a certain immutability, and cannot fall from that vision.

Hence it is said: Blessed are they who dwell in your house, O Lord: they shall praise you for ever and ever (Ps. lxxxiii, 5): He shall never be moved from his place, that dwelleth in Jerusalem (Ps. cxxiv, 1): Whoever shall overcome, I will make him a pillar in the temple of my God, and he shall not go out any more (Apoc. iii, 12).