Prayer is offered to a person in two ways: first, as to be fulfilled by him, secondly, as to be obtained through him. On the first way we offer prayer to God alone, since all our prayers ought to be directed to the acquisition of grace and glory, which God alone gives….
But in the second way we pray to the saints, whether angels or men, not that God may through them know our petitions, but that our prayers may be effective through their prayers and merits. Hence it is written that "the smoke of the incense”—namely, "the prayers of the saints”—"rose before God." This is also clear from the very style employed by the church in praying: since we beseech the blessed Trinity "to have mercy on us," while we ask any of the saints "to pray for us.”
ST. THOMAS AQUINAS Summa Theologiae
