But that the soul is made immortal is a further point in the Church's teaching which you must know, to show how the idols are to be overthrown. But we shall more directly arrive at a knowledge of this from what we know of the body, and from the difference between the body and the soul. For if our argument has proved it to be distinct from the body, while the body is by nature mortal, it follows that the soul is immortal, because it is not like the body. 2. And again, if as we have shown, the soul moves the body and is not moved by other things, it follows that the movement of the soul is spontaneous, and that this spontaneous movement goes on after the body is laid aside in the earth. If then the soul were moved by the body, it would follow that the severance of its motor would involve its death. But if the soul moves the body also, it follows all the more that it moves itself. But if moved by itself, it follows that it outlives the body. 3. For the movement of the soul is the same thing as its life, just as, of course, we call the body alive when it moves, and say that its death takes place when it ceases moving. But this can be made clearer once for all from the action of the soul in the body. For if even when united and coupled with the body it is not shut in or commensurate with the small dimensions of the body, but often, when the body lies in bed, not moving, but in death-like sleep, the soul keeps awake by virtue of its own power, and transcends the natural power of the body, and as though travelling away from the body while remaining in it, imagines and beholds things above the earth, and often even holds converse with the saints and angels who are above earthly and bodily existence, and approaches them in the confidence of the purity of its intelligence; shall it not all the more, when separated from the body at the time appointed by God Who coupled them together, have its knowledge of immortality more clear? For if even when coupled with the body it lived a life outside the body, much more shall its life continue after the death of the body, and live without ceasing by reason of God Who made it thus by His own Word, our Lord Jesus Christ. 4. For this is the reason why the soul thinks of and bears in mind things immortal and eternal, namely, because it is itself immortal. And just as, the body being mortal, its senses also have mortal things as their objects, so, since the soul contemplates and beholds immortal things, it follows that it is immortal and lives for ever. For ideas and thoughts about immortality never desert the soul, but abide in it, and are as it were the fuel in it which ensures its immortality. This then is why the soul has the capacity for beholding God, and is its own way thereto, receiving not from without but from herself the knowledge and apprehension of the Word of God.
34. The soul, then, if only it get rid of the stains of sin is able to know God directly, its own rational nature imaging back the Word of God, after whose image it was created. But even if it cannot pierce the cloud which sin draws over its vision, it is confronted by the witness of creation to God.
We repeat then what we said before, that just as men denied God, and worship things without soul, so also in thinking they have not a rational soul, they receive at once the punishment of their folly, namely, to be reckoned among irrational creatures: and so, since as though from lack of a soul of their own they superstitiously worship soulless gods, they are worthy of pity and guidance. 2. But if they claim to have a soul, and pride themselves on the rational principle, and that rightly, why do they, as though they had no soul, venture to go against reason, and think not as they ought, but make themselves out higher even than the Deity? For having a soul that is immortal and invisible to them, they make a likeness of God in things visible and mortal. Or why, in like manner as they have departed from God, do they not betake themselves to Him again? For they are able, as they turned away their understanding from God, and feigned as gods things that were not, in like manner to ascend with the intelligence of their soul, and turn back to God again. 3. But turn back they can, if they lay aside the filth of all lust which they have put on, and wash it away persistently, until they have got rid of all the foreign matter that has affected their soul, and can show it in its simplicity as it was made, that so they may be able by it to behold the Word of the Father after Whose likeness they were originally made. For the soul is made after the image and likeness of God, as divine Scripture also shows, when it says in the person of God: “Let us make man after our Image and likeness.” Whence also when it gets rid of all the filth of sin which covers it and retains only the likeness of the Image in its purity, then surely this latter being thoroughly brightened, the soul beholds as in a mirror the Image of the Father, even the Word, and by His means reaches the idea of the Father, Whose Image the Saviour is. 4. Or, if the soul's own teaching is insufficient, by reason of the external things which cloud its intelligence, and prevent its seeing what is higher, yet it is further possible to attain to the knowledge of God from the things which are seen, since Creation, as though in written characters, declares in a loud voice, by its order and harmony, its own Lord and Creator.
Source: Against the Heathen (New Advent)