Objection 1. It would seem that the saints in heaven do not pray for us. A man's action is more meritorious for himself than for others. But the saints in heaven do not merit for themselves, neither do they pray for themselves, since they are already established in the term. Neither therefore do they pray for us.
Objection 2. Further, the saints conform their will to God perfectly, so that they will only what God wills. Now what God wills is always fulfilled. Therefore it would be useless for the saints to pray for us.
Objection 3. Further, just as the saints in heaven are above, so are those in Purgatory, for they can no longer sin. Now those in Purgatory do not pray for us, on the contrary we pray for them. Therefore neither do the saints in heaven pray for us.
Objection 4. Further, if the saints in heaven pray for us, the prayers of the higher saints would be more efficacious; and so we ought not to implore the help of the lower saints' prayers but only of those of the higher saints.
Objection 5. Further, the soul of Peter is not Peter. If therefore the souls of the saints pray for us, so long as they are separated from their bodies, we ought not to call upon Saint Peter, but on his soul, to pray for us: yet the Church does the contrary. The saints therefore do not pray for us, at least before the resurrection.
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."
I answer that, As Jerome says (Cont. Vigilant. 6), the error of Vigilantius consisted in saying that "while we live, we can pray one for another; but that after we are dead, none of our prayers for others can be heard, seeing that not even the martyrs' prayers are granted when they pray for their blood to be avenged." But this is absolutely false, because, since prayers offered for others proceed from charity, as stated above (A7,8), the greater the charity of the saints in heaven, the more they pray for wayfarers, since the latter can be helped by prayers: and the more closely they are united to God, the more are their prayers efficacious: for the Divine order is such that lower beings receive an overflow of the excellence of the higher, even as the air receives the brightness of the sun. Wherefore it is said of Christ (Hebrews 7:25): "Going to God by His own power... to make intercession for us" [Vulgate: 'He is able to save for ever them that come to God by Him, always living to make intercession for us.']. Hence Jerome says (Cont. Vigilant. 6): "If the apostles and martyrs while yet in the body and having to be solicitous for themselves, can pray for others, how much more now that they have the crown of victory and triumph."
Reply to Objection 1. The saints in heaven, since they are blessed, have no lack of bliss, save that of the body's glory, and for this they pray. But they pray for us who lack the ultimate perfection of bliss: and their prayers are efficacious in impetrating through their previous merits and through God's acceptance.
Reply to Objection 2. The saints impetrate what ever God wishes to take place through their prayers: and they pray for that which they deem will be granted through their prayers according to God's will.
Reply to Objection 3. Those who are in Purgatory though they are above us on account of their impeccability, yet they are below us as to the pains which they suffer: and in this respect they are not in a condition to pray, but rather in a condition that requires us to pray for them.
Reply to Objection 4. It is God's will that inferior beings should be helped by all those that are above them, wherefore we ought to pray not only to the higher but also to the lower saints; else we should have to implore the mercy of God alone. Nevertheless it happens sometime that prayers addressed to a saint of lower degree are more efficacious, either because he is implored with greater devotion, or because God wishes to make known his sanctity.
Reply to Objection 5. It is because the saints while living merited to pray for us, that we invoke them under the names by which they were known in this life, and by which they are better known to us: and also in order to indicate our belief in the resurrection, according to the saying of Exodus 3:6, "I am the God of Abraham," etc.
Source: Summa Theologica (New Advent)