Objection 1. It seems that predestination cannot be furthered by the prayers of the saints. For nothing eternal can be preceded by anything temporal; and in consequence nothing temporal can help towards making something else eternal. But predestination is eternal. Therefore, since the prayers of the saints are temporal, they cannot so help as to cause anyone to become predestined. Predestination therefore is not furthered by the prayers of the saints.
Objection 2. Further, as there is no need of advice except on account of defective knowledge, so there is not need of help except through defective power. But neither of these things can be said of God when He predestines. Whence it is said: "Who hath helped the Spirit of the Lord? [Vulgate: 'Who hath known the mind of the Lord?'] Or who hath been His counsellor?" (Romans 11:34). Therefore predestination cannot be furthered by the prayers of the saints.
Objection 3. Further, if a thing can be helped, it can also be hindered. But predestination cannot be hindered by anything. Therefore it cannot be furthered by anything.
On the contrary, It is said that "Isaac besought the Lord for his wife because she was barren; and He heard him and made Rebecca to conceive" (Genesis 25:21). But from that conception Jacob was born, and he was predestined. Now his predestination would not have happened if he had never been born. Therefore predestination can be furthered by the prayers of the saints.
I answer that, Concerning this question, there were different errors. Some, regarding the certainty of divine predestination, said that prayers were superfluous, as also anything else done to attain salvation; because whether these things were done or not, the predestined would attain, and the reprobate would not attain, eternal salvation. But against this opinion are all the warnings of Holy Scripture, exhorting us to prayer and other good works.
Others declared that the divine predestination was altered through prayer. This is stated to have the opinion of the Egyptians, who thought that the divine ordination, which they called fate, could be frustrated by certain sacrifices and prayers. Against this also is the authority of Scripture. For it is said: "But the triumpher in Israel will not spare and will not be moved to repentance" (1 Samuel 15:29); and that "the gifts and the calling of God are without repentance" (Romans 11:29).
Wherefore we must say otherwise that in predestination two things are to be considered--namely, the divine ordination; and its effect. As regards the former, in no possible way can predestination be furthered by the prayers of the saints. For it is not due to their prayers that anyone is predestined by God. As regards the latter, predestination is said to be helped by the prayers of the saints, and by other good works; because providence, of which predestination is a part, does not do away with secondary causes but so provides effects, that the order of secondary causes falls also under providence. So, as natural effects are provided by God in such a way that natural causes are directed to bring about those natural effects, without which those effects would not happen; so the salvation of a person is predestined by God in such a way, that whatever helps that person towards salvation falls under the order of predestination; whether it be one's own prayers or those of another; or other good works, and such like, without which one would not attain to salvation. Whence, the predestined must strive after good works and prayer; because through these means predestination is most certainly fulfilled. For this reason it is said: "Labor more that by good works you may make sure your calling and election" (2 Peter 1:10).
Reply to Objection 1. This argument shows that predestination is not furthered by the prayers of the saints, as regards the preordination.
Reply to Objection 2. One is said to be helped by another in two ways; in one way, inasmuch as he receives power from him: and to be helped thus belongs to the weak; but this cannot be said of God, and thus we are to understand, "Who hath helped the Spirit of the Lord?" In another way one is said to be helped by a person through whom he carries out his work, as a master through a servant. In this way God is helped by us; inasmuch as we execute His orders, according to 1 Corinthians 3:9: "We are God's co-adjutors." Nor is this on account of any defect in the power of God, but because He employs intermediary causes, in order that the beauty of order may be preserved in the universe; and also that He may communicate to creatures the dignity of causality.
Reply to Objection 3. Secondary causes cannot escape the order of the first universal cause, as has been said above (Question 19, Article 6), indeed, they execute that order. And therefore predestination can be furthered by creatures, but it cannot be impeded by them.
Source: Summa Theologica (New Advent)