When I was going through the Rite of Christian Initiation for Adults (RCIA) process I was taught many things, by either the handouts, the various speakers, or my sponsor, that were—as I later discovered while doing my own research—were somewhat shaky as far as the teaching of the Church was concerned.
This is and will continue to be a problem for many Catholics for all time, but becoming familiar with what the Magisterium—the Church’s teaching authority—says is crucial to living life as a practicing Catholic; primarily because so many Catholics who proclaim to be acting within the Magisterium when pronouncing their views, are not.
The single best source is the Catechism of the Catholic Church, the online version from the Vatican.
Here is an excerpt from an article about the Magisterium by Thomas Storck.
“The crisis that has afflicted the Catholic Church since the middle of the 1960s has been a crisis of both faith and morals, that is, a crisis that has made many Catholics no longer know what to believe or what kind of conduct God expects of us. What is needed as a remedy for this is a firm standard, a reliable guide or teacher who can tell us both what we must believe and what we must do. And, of course, in Christ's true Church we do have such a reliable standard and guide. But even Catholics of good will can sometimes be confused about exactly which voices within the Church they are to follow.
“In the past the average Catholic could depend on the word of his parish priest if he had any doubts about correct Catholic belief or conduct, or even on the example of the many good Catholics about him. But today one can no longer trust everything that is said by just any priest or theologian, and our fellow parishioners are likely to be totally confused about what the Church proclaims to have been revealed by God. And so it behooves us to understand a word and concept that is apt to be unfamiliar or confusing. This word is Magisterium. Now the Latin word magisterium originally meant the duty or office of a teacher, tutor, master, etc. And in the case of the Church it means simply the teaching authority or office of the Church. The Magisterium is the teaching office of the Church, accomplished by the Holy Father and the bishops teaching in union with him.
“The rule of what we must believe as Catholics was defined by the First Vatican Council (1870) thus:
“. . . Further, all those things are to be believed with divine and Catholic faith which are contained in the Word of God, written or handed down, and which the Church, either by a solemn judgment or by her ordinary and universal teaching [magisterium], proposes for belief as having been divinely revealed.”
“This quotation brings up several points that must be explained. In the first place, the decree speaks of the "Word of God, written or handed down," that is, recorded either in Sacred Scripture or in Sacred Tradition. Now at first it might seem as if Sacred Scripture and Sacred Tradition are two separate sources of divine revelation. But the Second Vatican Council explained that in fact, "Sacred Tradition and Sacred Scripture make up a single sacred deposit of the Word of God, which is entrusted to the Church." In other words, the truths which God has revealed to his Church come to us through two modes, but they constitute one body of truth, the Word of God.
“Therefore the Protestant practice of equating the Word of God with only the written Bible is an error. Moreover, as should be obvious from a little reflection and historical knowledge, Sacred Scripture is itself a product of the Church's thought and activity, and in this sense a product of Sacred Tradition. This is true even though Scripture has God for its author and is itself a mode of revelation, for the human authors of the New Testament wrote from within the Church and took for granted the Church's teaching and worship as they wrote.
“The second point raised by the statement from the First Vatican Council is the distinction between the Church's extraordinary Magisterium and her ordinary and universal Magisterium, that is between what is taught "by a solemn judgment or by her ordinary and universal teaching." Thus the Magisterium operates via two methods. The solemn or extraordinary Magisterium is seen in solemn definitions either by a pope, as for example, the definition of the dogma of the Assumption of Mary into Heaven in 1950, or by one of the Ecumenical Councils of the Catholic Church ratified by the pope, as the definitions made by the Council of Trent (1545-1563) to reaffirm the Catholic faith against the Protestants or the definition of papal infallibility by the First Vatican Council in 1870.
“The ordinary and universal Magisterium, on the other hand, is the ordinary teaching of the Church, accomplished via papal pronouncements, statements of bishops, catechisms, homilies, etc. This is not to say that everything that any pope, bishop or priest has ever said on any occasion is part of the ordinary and universal Magisterium, but that it is via such means that this teaching is generally made known to the faithful. Note that the First Vatican Council speaks of it as both "ordinary and universal." "Ordinary" means that it is accomplished via the ordinary means of teaching that the Church uses, but "universal" means that it is taught by the entire body of bishops, and usually over a period of time. For generally when a doctrine has been taught as authoritative over time and by many popes and bishops, this indicates that it is a teaching of the ordinary and universal Magisterium and must be received and believed as faithfully as teaching that is solemnly defined by pope or council.” (italics in original)