The rule of faith is the teaching Church (the Magisterium) which safeguards and teaches to every generation God's revealed truth found in both Holy Scripture and Sacred Tradition.
This is not what Lad said. He said the Magisterium is the "role of" (not the rule of) faith. Not sure what "role of" means. Maybe he means the magisterium "has the role of (protecting the faith)." I would agree, but what if they DON'T protect the faith? This is where it gets complicated and you have to distinguish.
My point is asking what the definition of 'magisterium' is, is due to the fact that it is a new term, first used in the mid/late 1800s. And theologians have defined it in various ways, with various distinctions. So, much like the the 'papacy', or 'jurisdiction' which has various categories and levels, so does 'magisterium' need to be precisely explained, depending on what circuмstance you are talking about. There is the
ordinary magisterium (the current hierarchy) and there's the
UNIVERSAL magisterium (the consistent teachings of all the hierarchy's over the period of 2,000 years). The UNIVERSAL magisterium is also called multiple terms ('ordinary and universal' or 'perpetual').
Here's how I see it. The Church is based on Truth and Christ. Truth comes from Scripture/Tradition. Christ protects the Truth from OFFICIALLY being corrupted by the gates of hell, and the papacy (which takes the place of Christ on earth) is also charged with this job of protecting the Truth.
The magisterium is the OFFICIAL teachings of the Church, and it is simply the deciding factor in "what has always been taught" consistently for 2,000 years and is based on Scripture/Tradition. The popes from the past have clarified and ruled that this or that is consistent with Apostolic Truth. So, doctrine is our "rule of" faith. The magisterium (which is the pope, or the pope with the bishops, but HAS to include the pope, because he's the only one with the power of infallibility) has as its job, the duty to clarify and re-teach "what has always been taught".
The principle of 'Truth is authority' applies here. Truth is 1) scripture & tradition and 2) Dogmas which have been defined at councils and 3) any other truths (i.e. officially promulgated or not) which have been consistently taught for 2,000 years. A good example of #3 would be the truth that Our Lady is the 'mediatrix of all graces'. Many orthodox cardinals wanted to define this at V2 in hopes of countering the errors of protestants towards Our Lady, but they were thwarted. Even though it was not defined, this truth has been consistently held and is an implicit part of our faith.
If you hold that the 'magisterium' is the "rule of" faith, you could be right, if you SPECIFY that you are only talking about the UNIVERSAL magisterium, because this would include Scripture, Tradition and all doctrinal matters. But, if you're talking about the 'ordinary magisterium' being the "rule of" faith, then this would be backwards, because 1) the ordinary magisterium is just simply the current hierarchy and they are not infallible, unless they define something, per V1, and 2) this would be the false principle of 'whoever is in authority is Truth'. No, those in authority (i.e. hierarchy) do not determine Truth. This is what the modernists want us to think - that Truth changes based on who's in charge and 'the needs of the time'. No, Truth (which is what Drew means when he says 'dogma') is the ultimate authority, because truth comes from Christ, who is Truth itself.