Intercession of the saints
Intercession of the saints is the Catholic and Orthodox teaching that the saints in heaven intercede for the living on earth, just as the Hebrews asked prophets like Moses and David for intercession. The term intercession is used in the Christian church regarding the commending of oneself to the saints for assistance by their prayers to God. The Council of Trent declared in the decree to the bishops concerning instruction (Catechesis) that they teach the faithful that, "the saints, ruling together with Christ, offer their prayers for men to God, and that it is good and useful to invoke them suppliantly and to have recourse to their prayers and to their powerful help in obtaining benefits from God through Jesus Christ..." (Session 25)
The saints follow Christ in interceding for us with God the Father (Romans 8:34; Hebrews 7:25); also the act of dulia (reverence) is performed not to obtain from the saints but through their prayers what only God can bestow. The saints, knowing of man's needs and that Christ as Mediator bestows all favors of grace through the mediating ministry of Mary, His Mother, unite their own prayers with the prayers of Mary, direct them to Christ the Lord, and aid us as intermediaries.
This intercession of the saints is an instance in which the Church on earth and the Church in heaven (the Church Militant and the Church Triumphant), praying through Christ alone, acts for the salvation of mankind as well as for the individual members.
See Matthew 22:31-32; 2 Peter 1:15; Revelation 8:1-4; 20:4-5; also 2 Maccabees 15:11-16; Tobit 12:15.