The Church's authority has two pillars: The Word of God and the Magisterium of the Church. These two pillars are best represented in Jesus Christ, the living Word incarnate, and in the Virgin Mary, a figure of the Church as Jesus' Mother, who stand on either side of the Ark, like the pillars in Saint Bosco's dream.
The Word of God is Sacred Scripture and Sacred Tradition. In Scripture and in Tradition, God speaks to us; He tells us in Scripture that Jesus was born of a virgin in Nazareth and He tells us in Tradition that Mary was assumed into Heaven. Scripture and Tradition both speak the same word of God, because, there is but one God and one Word and because Scripture is from Tradition just as Jewish Scripture is from Jewish Tradition. This Word of God is so called because it is the sacred words which proceed from the sacred mouth of God, spoken by the living Word, centered on the same living Word, fulfilled by the same living Word, and is a unified word of instruction for us.
The Magisterium is the teaching authority of the Bishops of the Church, who, by their office, are teachers, as well as sanctifiers and governors, of the Church. The Church serves the Word of God: Her Magisterium serves the Word of God: She does not make doctrine but keeps, guards, gives, and preaches the doctrines of Jesus Christ, which are found in the Word of God. The Bishops, entrusted with the Word, live and preach it according to the episcopal office, and so they, our teachers, teach us the Catholic Faith. The Deposit of the Faith, the Faith, is the doctrines of Jesus found in the Word, so if we nourish ourselves on the Word, we will better know, and thus live by, the Faith, and if we do this we will also be nourished by Christ Himself in the Eucharist, for a Christian receives Him in Holy Communion, and since the Magisterium serves the Word and the Bishops and their co-operators the priests give us the Eucharist we will better know the unity of Magisterium and Word, Church and Christ, for these things are all one.
Because the Church is our teacher, as our mother, she has teaching authority, which I have just described, and because she has teaching authority she has the duty and the right to teach the Catholic Faith by her beliefs, her sacraments, her life, her prayers: Just as Mary teaches us good example, so the Church by her self - beliefs, sacraments, life and prayer -teaches us good example. Ergo, the Church, in virtue of her authority, has the duty and right to live by the Faith, to administer the sacraments, to live a Christian life, to pray and perform those rites which she has, such as exorcism, and in this way the Church discerns spirits, speaks on morals, ordains men: In short, she follows Jesus, she is herself, she enjoys the authority of God as a Divine Institution after the Divine Person, the same Jesus whom she follows, to whom all authority has been given by the Father.
Hence, we say it is by the authority of the Church that we believe the gospels, for apart from the Word and Magisterium, the gospels would not be believed, let alone been written, for they were written by the Church, by her first Bishops the Apostles, and they were believed by the Church as Scripture, and they were included in Scripture, and Scripture is a part of the Word of God, which in turn is a part of the Church's teaching authority. The unity of Church and Christ, Word and Magisterium is made clear in this way: The Church is the Body of Christ. His authority is hers, so whatever is in the Word is from Him and whatever the Bishops teach is from Him. Yet, we know, as the Word attests, that there are false shepherds and wolves who prowl the Church, not as necessary evils but as weeds among wheat, and we know that if they do not teach rightly, they do not teach truthfully, and so they ought to be corrected and prayed for, to the glory of Jesus. The Lord can transform weeds into wheat.
Because we believe the gospels by the Church's authority, some might claim we are fallacious, as if the Church lives by circular logic. Yet we do not believe that the gospels are from the Church, though written by her, for they are from Jesus, just as they are about Jesus, and He is the One from whom the Church receives her authority: So it is not a matter of "We believe this because the Church says this because we believe it" but "We believe this because the Church says this because Jesus says this". If not for God, Jesus would not be the Christ; if not for Jesus, the Church would not be the Divine Institution; if not for the Church, the work of Jesus would not be continued. Yet some only hold Jesus' miracles and Divinity as mere human beliefs, not facts, and so they accuse us of circular reasoning in saying the Church is from Jesus, for to them the Church recreated Jesus to suit and support her power, as if to say, "We believe this and that, so why not believe Jesus is God too for our authority" or "We need a source of authority, so let us make it Jesus". But for us who believe in God and His Messiah, we know that such thinking is fallacious.