Objection 1. It would seem that the prayers which the saints pour forth to God for us are not always granted. For if they were always granted, the saints would be heard especially in regard to matters concerning themselves. But they are not heard in reference to these things; wherefore it is stated in the Apocalypse (6:11) that on the martyrs beseeching vengeance on them that dwell on earth, "it was said to them that they should rest for a little while till the number of their brethren should be filled up [Vulgate: 'till their fellow-servants and their brethren... should be filled up']." Much less therefore, are they heard in reference to matters concerning others.
Objection 2. Further, it is written (Jeremiah 15:1): "If Moses and Samuel shall stand before Me, My soul is not towards this people." Therefore, the saints are not always heard when they pray God for us.
Objection 3. Further, the saints in heaven are stated to be equal to the angels of God (Matthew 22:30). But the angels are not always heard in the prayers which they offer up to God. This is evident from Daniel 10:12-13, where it is written: "I am come for thy words: but the prince of the kingdom of the Persians resisted me one-and-twenty days." But the angel who spoke had not come to Daniel's aid except by asking of God to be set free; and yet the fulfillment of his prayer was hindered. Therefore neither are other saints always heard by God when they pray for us.
Objection 4. Further, whosoever obtains something by prayer merits it in a sense. But the saints in heaven are not in the state of meriting. Therefore they cannot obtain anything for us from God by their prayers.
Objection 5. Further, the saints, in all things, conform their will to the will of God. Therefore they will nothing but what they know God to will. But no one prays save for what he wills. Therefore they pray not save for what they know God to will. Now that which God wills would be done even without their praying for it. Therefore their prayers are not efficacious for obtaining anything.
Objection 6. Further, the prayers of the whole heavenly court, if they could obtain anything, would be more efficacious than all the petitions of the Church here below. Now if the suffrages of the Church here below for some one in purgatory were to be multiplied, he would be wholly delivered from punishment. Since then the saints in heaven pray for those who are in purgatory on the same account as for us, if they obtain anything for us, their prayers would deliver entirely from punishment those who are in purgatory. But this is not true because, then the Church's suffrages for the dead would be unnecessary.
On the contrary, It is written (2 Maccabees 15:14): "This is he that prayeth much for the people, and for all the holy city, Jeremias the prophet of God": and that his prayer was granted is clear from what follows (2 Maccabees 15:15): "Jeremias stretched forth his right hand, and gave to Judas a sword of gold, saying: Take this holy sword, a gift from God," etc.
Further, Jerome says (Ep. contra Vigilant.): "Thou sayest in thy pamphlets, that while we live, we can pray for one another, but that when we are dead no one's prayer for another will be heard": and afterwards he refutes this in the following words: "If the apostles and martyrs while yet in the body can pray for others, while they are still solicitous for themselves, how much more can they do so when the crown, the victory, the triumph is already theirs!"
Further, this is confirmed by the custom of the Church, which often asks to be assisted by the prayers of the saints.
I answer that, The saints are said to pray for us in two ways. First, by "express" prayer, when by their prayers they seek a hearing of the Divine clemency on our behalf: secondly, by "interpretive" prayer, namely by their merits which, being known to God, avail not only them unto glory, but also us as suffrages and prayers, even as the shedding of Christ's blood is said to ask pardon for us. In both ways the saints' prayers considered in themselves avail to obtain what they ask, yet on our part they may fail so that we obtain not the fruit of their prayers, in so far as they are said to pray for us by reason of their merits availing on our behalf. But in so far as they pray for us by asking something for us in their prayers, their prayers are always granted, since they will only what God wills, nor do they ask save for what they will to be done; and what God wills is always fulfilled--unless we speak of His "antecedent" will, whereby "He wishes all men to be saved" [Cf. I, 19, 6, ad 1]. For this will is not always fulfilled; wherefore no wonder if that also which the saints will according to this kind of will be not fulfilled sometimes.
Reply to Objection 1. This prayer of the martyrs is merely their desire to obtain the robe of the body and the fellowship of those who will be saved, and their consent to God's justice in punishing the wicked. Hence a gloss on Apocalypse 6:11, "How long, O Lord," says: "They desire an increase of joy and the fellowship of the saints, and they consent to God's justice."
Reply to Objection 2. The Lord speaks there of Moses and Samuel according to their state in this life. For we read that they withstood God's anger by praying for the people. And yet even if they had been living at the time in question, they would have been unable to placate God towards the people by their prayers, on account of the wickedness of this same people: and it is thus that we are to understand this passage.
Reply to Objection 3. This dispute among the good angels does not mean that they offered contradictory prayers to God, but that they submitted contrary merits on various sides to the Divine inquiry, with a view of God's pronouncing sentence thereon. This, in fact, is what Gregory says (Moral. xvii) in explanation of the aforesaid words of Daniel: "The lofty spirits that are set over the nations never fight in behalf of those that act unjustly, but they justly judge and try their deeds. And when the guilt or innocence of any particular nation is brought into the debate of the court above, the ruling spirit of that nation is said to have won or lost in the conflict. Yet the supreme will of their Maker is victorious over all, for since they have it ever before their eyes, they will not what they are unable to obtain," wherefore neither do they seek for it. And consequently it is clear that their prayers are always heard.
Reply to Objection 4. Although the saints are not in a state to merit for themselves, when once they are in heaven, they are in a state to merit for others, or rather to assist others by reason of their previous merit: for while living they merited that their prayers should be heard after their death.
Or we may reply that prayer is meritorious on one count, and impetratory on another. For merit consists in a certain equation of the act to the end for which it is intended, and which is given to it as its reward; while the impetration of a prayer depends on the liberality of the person supplicated. Hence prayer sometimes, through the liberality of the person supplicated, obtains that which was not merited either by the suppliant, or by the person supplicated for: and so, although the saints are not in the state of meriting, it does not follow that they are not in the state of impetrating.
Reply to Objection 5. As appears from the authority of Gregory quoted above (ad 3), the saints and angels will nothing but what they see to be in the Divine will: and so neither do they pray for aught else. Nor is their prayer fruitless, since as Augustine says (De Praed. Sanct. [De Dono Persever. xxii]): "The prayers of the saints profit the predestinate, because it is perhaps pre-ordained that they shall be saved through the prayers of those who intercede for them": and consequently God also wills that what the saints see Him to will shall be fulfilled through their prayers.
Reply to Objection 6. The suffrages of the Church for the dead are as so many satisfactions of the living in lieu of the dead: and accordingly they free the dead from the punishment which the latter have not paid. But the saints in heaven are not in the state of making satisfaction; and consequently the parallel fails between their prayers and the suffrages of the Church.
Source: Summa Theologica (New Advent)