Objection 1. It would seem that we ought not to call upon the saints to pray for us. For no man asks anyone's friends to pray for him, except in so far as he believes he will more easily find favor with them. But God is infinitely more merciful than any saint, and consequently His will is more easily inclined to give us a gracious hearing, than the will of a saint. Therefore it would seem unnecessary to make the saints mediators between us and God, that they may intercede for us.
Objection 2. Further, if we ought to beseech them to pray for us, this is only because we know their prayer to be acceptable to God. Now among the saints the holier a man is, the more is his prayer acceptable to God. Therefore we ought always to bespeak the greater saints to intercede for us with God, and never the lesser ones.
Objection 3. Further, Christ, even as man, is called the "Holy of Holies," and, as man, it is competent to Him to pray. Yet we never call upon Christ to pray for us. Therefore neither should we ask the other saints to do so.
Objection 4. Further, whenever one person intercedes for another at the latter's request, he presents his petition to the one with whom he intercedes for him. Now it is unnecessary to present anything to one to whom all things are present. Therefore it is unnecessary to make the saints our intercessors with God.
Objection 5. Further, it is unnecessary to do a thing if, without doing it, the purpose for which it is done would be achieved in the same way, or else not achieved at all. Now the saints would pray for us just the same, or would not pray for us at all, whether we pray to them or not: for if we be worthy of their prayers, they would pray for us even though we prayed not to them, while if we be unworthy they pray not for us even though we ask them to. Therefore it seems altogether unnecessary to call on them to pray for us.
On the contrary, It is written (Job 5:1): "Call... if there be any that will answer thee, and turn to some of the saints." Now, as Gregory says (Moral. v, 30) on this passage, "we call upon God when we beseech Him in humble prayer." Therefore when we wish to pray God, we should turn to the saints, that they may pray God for us.
Further, the saints who are in heaven are more acceptable to God than those who are on the way. Now we should make the saints, who are on the way, our intercessors with God, after the example of the Apostle, who said (Romans 15:30): "I beseech you... brethren, through our Lord Jesus Christ, and by the charity of the Holy Ghost, that you help me in your prayers for me to God." Much more, therefore, should we ask the saints who are in heaven to help us by their prayers to God.
Further, an additional argument is provided by the common custom of the Church which asks for the prayers of the saints in the Litany.
I answer that, According to Dionysius (Eccl. Hier. v) the order established by God among things is that "the last should be led to God by those that are midway between." Wherefore, since the saints who are in heaven are nearest to God, the order of the Divine law requires that we, who while we remain in the body are pilgrims from the Lord, should be brought back to God by the saints who are between us and Him: and this happens when the Divine goodness pours forth its effect into us through them. And since our return to God should correspond to the outflow of His boons upon us, just as the Divine favors reach us by means of the saints intercession, so should we, by their means, be brought back to God, that we may receive His favors again. Hence it is that we make them our intercessors with God, and our mediators as it were, when we ask them to pray for us.
Reply to Objection 1. It is not on account of any defect in God's power that He works by means of second causes, but it is for the perfection of the order of the universe, and the more manifold outpouring of His goodness on things, through His bestowing on them not only the goodness which is proper to them, but also the faculty of causing goodness in others. Even so it is not through any defect in His mercy, that we need to bespeak His clemency through the prayers of the saints, but to the end that the aforesaid order in things be observed.
Reply to Objection 2. Although the greater saints are more acceptable to God than the lesser, it is sometimes profitable to pray to the lesser; and this for five reasons. First, because sometimes one has greater devotion for a lesser saint than for a greater, and the effect of prayer depends very much on one's devotion. Secondly, in order to avoid tediousness, for continual attention to one thing makes a person weary; whereas by praying to different saints, the fervor of our devotion is aroused anew as it were. Thirdly, because it is granted to some saints to exercise their patronage in certain special cases, for instance to Saint Anthony against the fire of hell. Fourthly, that due honor be given by us to all. Fifthly, because the prayers of several sometimes obtain that which would not have been obtained by the prayers of one.
Reply to Objection 3. Prayer is an act, and acts belong to particular persons [supposita]. Hence, were we to say: "Christ, pray for us," except we added something, this would seem to refer to Christ's person, and consequently to agree with the error either of Nestorius, who distinguished in Christ the person of the son of man from the person of the Son of God, or of Arius, who asserted that the person of the Son is less than the Father. Wherefore to avoid these errors the Church says not: "Christ, pray for us," but "Christ, hear us," or "have mercy on us."
Reply to Objection 4. As we shall state further on (3) the saints are said to present our prayers to God, not as though they notified things unknown to Him, but because they ask God to grant those prayers a gracious hearing, or because they seek the Divine truth about them, namely what ought to be done according to His providence.
Reply to Objection 5. A person is rendered worthy of a saint's prayers for him by the very fact that in his need he has recourse to him with pure devotion. Hence it is not unnecessary to pray to the saints.
Source: Summa Theologica (New Advent)