Objection 1. It would seem that the saints, seeing God in His essence, see all that God sees in Himself. For as Isidore says (De Sum. Bon. 1.): "The angels know all things in the World of God, before they happen." Now the saints will be equal to the angels of God (Matthew 22:30). Therefore the saints also in seeing God see all things.
Objection 2. Further, Gregory says (Dial. iv.): "Since all see God there with equal clearness, what do they not know, who know Him Who knows all things?" and he refers to the blessed who see God in His essence. Therefore those who see God in His essence know all things.
Objection 3. Further, it is stated in De Anima (iii, text. 7), that "when an intellect understands the greatest things, it is all the more able to understand the least things." Now God is the greatest of intelligible things. Therefore the power of the intellect is greatly increased by understanding Him. Therefore the intellect seeing Him understands all things.
Objection 4. Further, the intellect is not hindered from understanding a thing except by this surpassing it. Now no creature surpasses the intellect that understands God, since, as Gregory says (Dial. ii.), "to the soul which sees its Creator all creatures are small." Therefore those who see God in His essence know all things.
Objection 5. Further, every passive power that is not reduced to act is imperfect. Now the passive intellect of the human soul is a power that is passive as it were to the knowledge of all things, since "the passive intellect is in which all are in potentiality" (De Anima iii, text. 18). If then in that beatitude it were not to understand all things, it would remain imperfect, which is absurd.
Objection 6. Further, whoever sees a mirror sees the things reflected in the mirror. Now all things are reflected in the Word of God as in a mirror, because He is the type and image of all. Therefore the saints who see the Word in its essence see all created things.
Objection 7. Further, according to Proverbs 10:24, "to the just their desire shall be given." Now the just desire to know all things, since "all men desire naturally to know," and nature is not done away by glory. Therefore God will grant them to know all things.
Objection 8. Further, ignorance is one of the penalties of the present life [Cf. I-II, 85, 3]. Now all penalty will be removed from the saints by glory. Therefore all ignorance will be removed: and consequently they will know all.
Objection 9. Further, the beatitude of the saints is in their soul before being in their body. Now the bodies of the saints will be reformed in glory to the likeness of Christ's body (Philippians 3:21). Therefore their souls will be perfected in likeness to the soul of Christ. Now Christ's soul sees all things in the Word. Therefore all the souls of the saints will also see all things in the Word.
Objection 10. Further, the intellect, like the senses, knows all the things with the image of which it is informed. Now the Divine essence shows a thing forth more clearly than any other image thereof. Therefore since in that blessed vision the Divine essence becomes the form as it were of our intellect, it would seem that the saints seeing God see all.
Objection 11. Further, the Commentator says (De Anima iii), that "if the active intellect were the form of the passive intellect, we should understand all things." Now the Divine essence represents all things more clearly than the active intellect. Therefore the intellect that sees God in His essence knows all things.
Objection 12. Further, the lower angels are enlightened by the higher about the things they are ignorant of, for the reason that they know not all things. Now after the day of judgment, one angel will not enlighten another; for then all superiority will cease, as a gloss observes on 1 Corinthians 15:24, "When He shall have brought to nought," etc. Therefore the lower angels will then know all things, and for the same reason all the other saints who will see God in His essence.
On the contrary, Dionysius says (Hier. Eccles. vi): "The higher angels cleanse the lower angels from ignorance." Now the lower angels see the Divine essence. Therefore an angel while seeing the Divine essence may be ignorant of certain things. But the soul will not see God more perfectly than an angel. Therefore the souls seeing God will not necessarily see all things.
Further, Christ alone has the spirit not "by measure" (John 3:34). Now it becomes Christ, as having the spirit without measure, to know all things in the Word: wherefore it is stated in the same place (John 3:35) that "the Father... hath given all things into His hand." Therefore none but Christ is competent to know all things in the Word.
Further, the more perfectly a principle is known, the more of its effects are known thereby. Now some of those who see God in His essence will know God more perfectly than others. Therefore some will know more things than others, and consequently every one will not know all.
I answer that, God by seeing his essence knows all things whatsoever that are, shall be, or have been: and He is said to know these things by His "knowledge of vision," because He knows them as though they were present in likeness to corporeal vision. Moreover by seeing this essence He knows all that He can do, although He never did them, nor ever will: else He would not know His power perfectly; since a power cannot be known unless its objects be known: and this is called His "science" or "knowledge of simple intelligence." Now it is impossible for a created intellect, by seeing the Divine essence, to know all that God can do, because the more perfectly a principle is known, the more things are known in it; thus in one principle of demonstration one who is quick of intelligence sees more conclusions than one who is slow of intelligence. Since then the extent of the Divine power is measured according to what it can do, if an intellect were to see in the Divine essence all that God can do, its perfection in understanding would equal in extent the Divine power in producing its effects, and thus it would comprehend the Divine power, which is impossible for any created intellect to do. Yet there is a created intellect, namely the soul of Christ [Cf. III, 16, 2], which knows in the Word all that God knows by the knowledge of vision. But regarding others who see the Divine essence there are two opinions. For some say that all who see God in His essence see all that God sees by His knowledge of vision. This, however, is contrary to the sayings of holy men, who hold that angels are ignorant of some things; and yet it is clear that according to faith all the angels see God in His essence. Wherefore others say that others than Christ, although they see God in His essence, do not see all that God sees because they do not comprehend the Divine essence. For it is not necessary that he who knows a cause should know all its effects, unless he comprehend the cause: and this is not in the competency of a created intellect. Consequently of those who see God in His essence, each one sees in His essence so much the more things according as he sees the Divine essence the more clearly: and hence it is that one is able to instruct another concerning these things. Thus the knowledge of the angels and of the souls of the saints can go on increasing until the day of judgment, even as other things pertaining to the accidental reward. But afterwards it will increase no more, because then will be the final state of things, and in that state it is possible that all will know everything that God knows by the knowledge of vision.
Reply to Objection 1. The saying of Isidore, that "the angels know in the Word all things before they happen," cannot refer to those things which God knows only by the knowledge of simple intelligence, because those things will never happen; but it must refer to those things which God knows only by the knowledge of vision. Even of these he does not say that all the angels know them all, but that perhaps some do; and that even those who know do not know all perfectly. For in one and the same thing there are many intelligible aspects to be considered, such as its various properties and relations to other things: and it is possible that while one thing is known in common by two persons, one of them perceives more aspects, and that the one learns these aspects from the other. Hence Dionysius says (Div. Nom. iv) that "the lower angels learn from the higher angels the intelligible aspects of things." Wherefore it does not follow that even the angels who know all creatures are able to see all that can be understood in them.
Reply to Objection 2. It follows from this saying of Gregory that this blessed vision suffices for the seeing of all things on the part of the Divine essence, which is the medium by which one sees, and whereby God sees all things. That all things, however, are not seen is owing to the deficiency of the created intellect which does not comprehend the Divine essence.
Reply to Objection 3. The created intellect sees the Divine essence not according to the mode of that same essence, but according to its own mode which is finite. Hence its efficacy in knowing would need to be infinitely increased by reason of that vision in order for it to know all things.
Reply to Objection 4. Defective knowledge results not only from excess and deficiency of the knowable object in relation to the intellect, but also from the fact that the aspect of knowableness is not united to the intellect: thus sometimes the sight sees not a stone, through the image of the stone not being united to it. And although the Divine essence which is the type of all things is united to the intellect of one who sees God, it is united thereto not as the type of all things, but as the type of some and of so much the more according as one sees the Divine essence more fully.
Reply to Objection 5. When a passive power is perceptible by several perfections in order, if it be perfected with its ultimate perfection, it is not said to be imperfect, even though it lack some of the preceding dispositions. Now all knowledge by which the created intellect is perfected is directed to the knowledge of God as its end. Wherefore he who sees God in His essence, even though he know nothing else, would have a perfect intellect: nor is his intellect more perfect through knowing something else besides Him, except in so far as it sees Him more fully. Hence Augustine says (Confess. v.): "Unhappy is he who knoweth all these" (namely, creatures), "and knoweth not Thee: but happy whoso knoweth Thee, though he know not these. And whoso knoweth both Thee and them is not the happier for them but for Thee only."
Reply to Objection 6. This mirror has a will: and even as He will show Himself to whom He will, so will He show in Himself whatsoever He will. Nor does the comparison with a material mirror hold, for it is not in its power to be seen or not to be seen.
We may also reply that in a material mirror both object and mirror are seen under their proper image; although the mirror be seen through an image received from the thing itself, whereas the stone is seen through its proper image reflected in some other thing, where the reason for seeing the one is the reason for seeing the other. But in the uncreated mirror a thing is seen through the form of the mirror, just as an effect is seen through the image of its cause and conversely. Consequently it does not follow that whoever sees the eternal mirror sees all that is reflected in that mirror: since he who sees the cause does not of necessity see all its effects, unless he comprehend the cause.
Reply to Objection 7. The desire of the saints to know all things will be fulfilled by the mere fact of their seeing God: just as their desire to possess all good things will be fulfilled by their possessing God. For as God suffices the affections in that He has perfect goodness, and by possessing Him we possess all goods as it were, so does the vision of Him suffice the intellect: "Lord, show us the Father and it is enough for us" (John 14:8).
Reply to Objection 8. Ignorance properly so called denotes a privation and thus it is a punishment: for in this way ignorance is nescience of things, the knowledge of which is a duty or a necessity. Now the saints in heaven will not be ignorant of any of these things. Sometimes, however, ignorance is taken in a broad sense of any kind of nescience: and thus the angels and saints in heaven will be ignorant of certain things. Hence Dionysius says (Div. Nom. iv) that "the angels will be cleansed from their ignorance." In this sense ignorance is not a penalty but a defect. Nor is it necessary for all such defects to be done away by glory: for thus we might say that it was a defect in Pope Linus that he did not attain to the glory of Peter.
Reply to Objection 9. Our body will be conformed to the body of Christ in glory, in likeness but not in equality, for it will be endowed with clarity even as Christ's body, but not equally. In like manner our soul will have glory in likeness to the soul of Christ, but not in equality thereto: thus it will have knowledge even as Christ's soul, but not so great, so as to know all as Christ's soul does.
Reply to Objection 10. Although the Divine essence is the type of all things knowable it will not be united to each created intellect according as it is the type of all. Hence the objection proves nothing.
Reply to Objection 11. The active intellect is a form proportionate to the passive intellect; even as the passive power of matter is proportionate to the power of the natural agent, so that whatsoever is in the passive power of matter or the passive intellect is in the active power of the active intellect or of the natural agent. Consequently if the active intellect become the form of the passive intellect, the latter must of necessity know all those things to which the power of the active intellect extends. But the Divine essence is not a form proportionate to our intellect in this sense. Hence the comparison fails.
Reply to Objection 12. Nothing hinders us from saying that after the judgment day, when the glory of men and angels will be consummated once for all, all the blessed will know all that God knows by the knowledge of vision, yet so that not all will see all in the Divine essence. Christ's soul, however, will see clearly all things therein, even as it sees them now; while others will see therein a greater or lesser number of things according to the degree of clearness wherewith they will know God: and thus Christ's soul will enlighten all other souls concerning those things which it sees in the Word better than others. Hence it is written (Apocalypse 21:23): "The glory of God shall enlighten the city of Jerusalem [Vulgate: 'hath enlightened it'], and the Lamb is the lamp thereof." In like manner the higher souls will enlighten the lower (not indeed with a new enlightening, so as to increase the knowledge of the lower), but with a kind of continued enlightenment; thus we might understand the sun to enlighten the atmosphere while at a standstill. Wherefore it is written (Daniel 12:3): "They that instruct many to justice" shall shine "as stars for all eternity." The statement that the superiority of the orders will cease refers to their present ordinate ministry in our regard, as is clear from the same gloss.
Source: Summa Theologica (New Advent)