Why is the magisterium important?

Answer 1

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Catholic AnswerThe Magisterium is the teaching authority of the Church, it is the ability of the Bishops, always in union with the Pope, to infallibly teach Christian doctrine. The Despositium Fidei is the Latin title for the Deposit of Faith, which is what the Magisterium teaches, it is the faith which was entrusted to the Church to teach. It is contained in Sacred Tradition, and also found in the Sacred Scriptures, although the New Testament is an example of the Magisterium putting the Depositium Fidei infallibly down on paper (or parchment or vellum), as, quite obviously, the Depositium Fidei was here when Our Blessed Lord entrusted it to the apostles, and His first Vicar - St. Peter; while the New Testament would not exist as such for another four centuries.

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So, the short answer is the Magisterium is the teacher, and the Despoitium Fidei is that which is taught, together they are the Teaching Church.