Objection 1. It would seem that prayer is not meritorious. All merit proceeds from grace. But prayer precedes grace, since even grace is obtained by means of prayer according to Luke 11:13, "(How much more) will your Father from heaven give the good Spirit to them that ask Him!" Therefore prayer is not a meritorious act.
Objection 2. Further, if prayer merits anything, this would seem to be chiefly that which is besought in prayer. Yet it does not always merit this, because even the saints' prayers are frequently not heard; thus Paul was not heard when he besought the sting of the flesh to be removed from him. Therefore prayer is not a meritorious act.
Objection 3. Further, prayer is based chiefly on faith, according to James 1:6, "But let him ask in faith, nothing wavering." Now faith is not sufficient for merit, as instanced in those who have lifeless faith. Therefore prayer is not a meritorious act.
On the contrary, A gloss on the words of Psalm 34:13, "My prayer shall be turned into my bosom," explains them as meaning, "if my prayer does not profit them, yet shall not I be deprived of my reward." Now reward is not due save to merit. Therefore prayer is meritorious.
I answer that, As stated above (Article 13) prayer, besides causing spiritual consolation at the time of praying, has a twofold efficacy in respect of a future effect, namely, efficacy in meriting and efficacy in impetrating. Now prayer, like any other virtuous act, is efficacious in meriting, because it proceeds from charity as its root, the proper object of which is the eternal good that we merit to enjoy. Yet prayer proceeds from charity through the medium of religion, of which prayer is an act, as stated above (Article 3), and with the concurrence of other virtues requisite for the goodness of prayer, viz. humility and faith. For the offering of prayer itself to God belongs to religion, while the desire for the thing. that we pray to be accomplished belongs to charity. Faith is necessary in reference to God to Whom we pray; that is, we need to believe that we can obtain from Him what we seek. Humility is necessary on the part of the person praying, because he recognizes his neediness. Devotion too is necessary: but this belongs to religion, for it is its first act and a necessary condition of all its secondary acts, as stated above (82, 1 and 2).
As to its efficacy in impetrating, prayer derives this from the grace of God to Whom we pray, and Who instigates us to pray. Wherefore Augustine says (De Verb. Dom., Serm. cv, 1): "He would not urge us to ask, unless He were willing to give"; and Chrysostom says: "He never refuses to grant our prayers, since in His loving-kindness He urged us not to faint in praying."
Reply to Objection 1. Neither prayer nor any other virtuous act is meritorious without sanctifying grace. And yet even that prayer which impetrates sanctifying grace proceeds from some grace, as from a gratuitous gift, since the very act of praying is "a gift of God," as Augustine states (De Persever. xxiii).
Reply to Objection 2. Sometimes the merit of prayer regards chiefly something distinct from the object of one's petition. For the chief object of merit is beatitude, whereas the direct object of the petition of prayer extends sometimes to certain other things, as stated above (A6,7). Accordingly if this other thing that we ask for ourselves be not useful for our beatitude, we do not merit it; and sometimes by asking for and desiring such things we lose merit for instance if we ask of God the accomplishment of some sin, which would be an impious prayer. And sometimes it is not necessary for salvation, nor yet manifestly contrary thereto; and then although he who prays may merit eternal life by praying, yet he does not merit to obtain what he asks for. Hence Augustine says (Liber. Sentent. Prosperi sent. ccxii): "He who faithfully prays God for the necessaries of this life, is both mercifully heard, and mercifully not heard. For the physician knows better than the sick man what is good for the disease." For this reason, too, Paul was not heard when he prayed for the removal of the sting in his flesh, because this was not expedient. If, however, we pray for something that is useful for our beatitude, through being conducive to salvation, we merit it not only by praying, but also by doing other good deeds: therefore without any doubt we receive what we ask for, yet when we ought to receive it: "since certain things are not denied us, but are deferred that they may be granted at a suitable time," according to Augustine (Tract. cii in Joan.): and again this may be hindered if we persevere not in asking for it. Wherefore Basil says (De Constit. Monast. i): "The reason why sometimes thou hast asked and not received, is because thou hast asked amiss, either inconsistently, or lightly, or because thou hast asked for what was not good for thee, or because thou hast ceased asking." Since, however, a man cannot condignly merit eternal life for another, as stated above (I-II, 114, 6), it follows that sometimes one cannot condignly merit for another things that pertain to eternal life. For this reason we are not always heard when we pray for others, as stated above (07, ad 2,3). Hence it is that four conditions are laid down; namely, to ask--"for ourselves--things necessary for salvation--piously--perseveringly"; when all these four concur, we always obtain what we ask for.
Reply to Objection 3. Prayer depends chiefly on faith, not for its efficacy in meriting, because thus it depends chiefly on charity, but for its efficacy in impetrating, because it is through faith that man comes to know of God's omnipotence and mercy, which are the source whence prayer impetrates what it asks for.
Source: Summa Theologica (New Advent)