Objection 1. It would seem that the time of the future judgment is not unknown. For just as the holy Fathers looked forward to the first coming, so do we look forward to the second. But the holy Fathers knew the time of the first coming, as proved by the number of weeks mentioned in Daniel 9: wherefore the Jews are reproached for not knowing the time of Christ's coming (Luke 12:56): "You hypocrites, you know how to discern the face of the heaven and of the earth, but how is it that you do not discern this time?" Therefore it would seem that the time of the second coming when God will come to judgment should also be certified to us.
Objection 2. Further, we arrive by means of signs at the knowledge of the things signified. Now many signs of the coming judgment are declared to us in Scripture (Matthew 24, Mark 13, Luke 21). Therefore we can arrive at the knowledge of that time.
Objection 3. Further, the Apostle says (1 Corinthians 10:11): "It is on us ['These things... are written for our correction, upon whom the ends of the world are come'] that the ends of the world are come," and (1 John 2:18): "Little children, it is the last hour," etc. Since then it is a long time since these things were said, it would seem that now at least we can know that the last judgment is nigh.
Objection 4. Further, there is no need for the time of the judgment to be hidden, except that each one may be careful to prepare himself for judgment, being in ignorance of the appointed time. Yet the same care would still be necessary even were the time known for certain, because each one is uncertain about the time of his death, of which Augustine says (Ep. ad Hesych. cxcix) that "as each one's last day finds him, so will the world's last day find him." Therefore there is no necessity for the time of the judgment to be uncertain.
On the contrary, It is written (Mark 13:32): "Of that day or hour no man knoweth, neither the angels in heaven, nor the Son, but the Father." The Son, however, is said not to know in so far as He does not impart the knowledge to us.
Further, it is written (1 Thessalonians 5:2): "The day of the Lord shall so come as a thief in the night." Therefore seemingly, as the coming of a thief in the night is altogether uncertain, the day of the last judgment is altogether uncertain.
I answer that, God is the cause of things by His knowledge [Cf. I, 14, 8]. Now He communicates both these things to His creatures, since He both endows some with the power of action on others whereof they are the cause, and bestows on some the knowledge of things. But in both cases He reserves something to Himself, for He operates certain things wherein no creature co-operates with Him, and again He knows certain things which are unknown to any mere creature. Now this should apply to none more than to those things which are subject to the Divine power alone, and in which no creature co-operates with Him. Such is the end of the world when the day of judgment will come. For the world will come to an end by no created cause, even as it derived its existence immediately from God. Wherefore the knowledge of the end of the world is fittingly reserved to God. Indeed our Lord seems to assign this very reason when He said (Acts 1:7): "It is not for you to know the times or moments which the Father hath put in His own power," as though He were to say, "which are reserved to His power alone."
Reply to Objection 1. At His first coming Christ came secretly according to Isaiah 45:15, "Verily Thou art a hidden God, the God of Israel, the Saviour." Hence, that He might be recognized by believers, it was necessary for the time to be fixed beforehand with certainty. On the other hand, at the second coming, He will come openly, according to Psalm 49:3, "God shall come manifestly." Consequently there can be no error affecting the knowledge of His coming. Hence the comparison fails.
Reply to Objection 2. As Augustine says, in his letter to Hesychius concerning the day of judgment (Ep. cxcix), "the signs mentioned in the Gospels do not all refer to the second advent which will happen at the end of the world, but some of them belong to the time of the sack of Jerusalem, which is now a thing of the past, while some, in fact many of them, refer to the advent whereby He comes daily to the Church, whom He visits spiritually when He dwells in us by faith and love." Moreover, the details mentioned in the Gospels and Epistles in connection with the last advent are not sufficient to enable us to determine the time of the judgment, for the trials that are foretold as announcing the proximity of Christ's coming occurred even at the time of the Early Church, in a degree sometimes more sometimes less marked; so that even the days of the apostles were called the last days (Acts 2:17) when Peter expounded the saying of Joel 2:28, "It shall come to pass in the last days," etc., as referring to that time. Yet it was already a long time since then: and sometimes there were more and sometimes less afflictions in the Church. Consequently it is impossible to decide after how long a time it will take place, nor fix the month, year, century, or thousand years as Augustine says in the same book (Ep. ad Hesych. cxcix). And even if we are to believe that at the end these calamities will be more frequent, it is impossible to fix what amount of such calamities will immediately precede the judgment day or the coming of Antichrist, since even at the time of the Early Church persecutions were so bitter, and the corruptions of error were so numerous, that some looked forward to the coming of Antichrist as being near or imminent; as related in Eusebius' History of the Church (vi, 7) and in Jerome's book De Viris Illustribus lii.
Reply to Objection 3. The statement, "It is the last hour" and similar expressions that are to be found in Scripture do not enable us to know the exact length of time. For they are not intended to indicate a short length of time, but to signify the last state of the world, which is the last age of all, and it is not stated definitely how long this will last. Thus neither is fixed duration appointed to old age, which is the last age of man, since sometimes it is seen to last as long as or even longer than all the previous ages, as Augustine remarks (Qq. 83, qu. lviii). Hence also the Apostle (2 Thessalonians 2:2) disclaims the false signification which some had given to his words, by believing that the day of the Lord was already at hand.
Reply to Objection 4. Notwithstanding the uncertainty of death, the uncertainty of the judgment conduces to watchfulness in two ways. First, as regards the thing ignored, since its delay is equal to the length of man's life, so that on either side uncertainty provokes him to greater care. Secondly, for the reason that a man is careful not only of his own person, but also of his family, or of his city or kingdom, or of the whole Church, the length of whose duration is not dependent on the length of man's life. And yet it behooves each of these to be so ordered that the day of the Lord find us not unprepared.
Source: Summa Theologica (New Advent)