The Catechism of the Catholic Church affirms this in numerous places. For example, no. 771 reads:
“The one mediator, Christ, established and ever sustains here on earth his holy Church, the community of faith, hope, and charity, as a visible organization through which he communicates truth and grace to all men.” (citing another official Catholic document from Vatican II, Lumen gentium, 8 § 1.).
The Council of Trent stated:
“If anyone asserts that this sin of Adam, which in its origin is one, and by propagation, not by imitation, transfused into all, which is in each one as something that is his own, is taken away either by the forces of human nature or by a remedy other than the merit of the one mediator, our Lord Jesus Christ . . . let him be anathema.” (Session V, 3).
Scriptural Facts I:
- Christ is the only Mediator between men and God. (1 Timothy 2:5)
- The Church is the Body of Christ, in which Jesus is the Head. ( Colossians 1:18)
- Jesus’ Eternal Priesthood. (Hebrews 7: 24)
The Catholic Answer:
Catholics do not argue with either of the facts stated above; these are Scriptural Truths. However, the Church reads the Scriptures as a whole and interprets them as ONE book.
The Catholic understanding is that, although Christ is the only Mediator. The Church, as Member of His Body, is also part of this ‘One Mediator’ and in “all things grow up into him who is the Head, that is, Christ. From him the whole body, joined and held together by every supporting ligament, grows and builds itself up in love, as each part does its work”. (Ephesians 4:15-16)
Jesus is the Eternal High Priest and the Church, as members of His Body, shares in that priesthood. This is why St Peter, in a clear reference to the Old Testament Priesthood - where priests were intercessors between God and men - could declare that we are built up a holy priesthood (1 Peter 2:5 and 2:9), as we are joined to Him as members of His Body. The book of Revelation 1:6 says that we “are a kingdom, priests to his God and Father“. As the Mediator, Jesus is not alone, He is the head of the Church in which all members can intercede for one another through Him.
This is the notion of Totus Christus, a Biblical Theology of the Whole Christ which is confirmed in the Scriptural account of Paul’s conversion in Acts 9:4, where Jesus asks Paul, then Saul: “Saul, Saul, why are you persecuting me?” and not “Why are you persecuting my followers?” - His Church.
What about the deceased Saints?
We’ve seen that the living members of the Church can indeed pray for one another, but how can we explain the intercession of the Saints? The Catholic Church teaches about the Communion of the Saints, that is, the entire Church is One in Christ. The deceased members of the Body of Christ are not separated from Christ once they leave their physical bodies, because “if Christ is in you, although the body is dead because of sin, the Spirit is life because of righteousness” (Rom 8:10). As St Paul states it “neither death nor life [...] will be able to separate us from the love of God in Christ Jesus our Lord.” (Rom 8: 38-39). Therefore, the souls of the Saints departed never cease to be part of His Body, they are part of the Church in Heaven, the Church Triumphant.
Once in Heaven, the Saints can intercede for the members of the Church on earth, the Church Militant. The Bible confirms that the Saints in Heaven do pray to God:
When he opened the fifth seal, I saw under the altar the souls of those who had been slain for the word of God and for the witness they had borne. They cried out with a loud voice, “O Sovereign Lord, holy and true, how long before you will judge and avenge our blood on those who dwell on the earth?” (Rev 6: 9-10)
For more on the Saints read: Why do Catholics pray to Saints?