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 …

Objection 2: Further, as there is no need of advice except on account of defective knowledge, so there is no 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? Or who hath been His counsellor?" (Rom. 11:34) …

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" (Gen. 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 …

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 Pet. 1:10).

—from Summa Theologica, Part I, Article 23;Translated by Fathers of the English Dominican Province (Benziger Bros., 1947)