We must always have an external standard to determine the accuracy of our thinking.
We cannot get sucked into the culture of relativism, modernism, and immanantism which hold that there is no such this as absolute truth, and that our standard of truth is found within us (based on our own fickle, whimsical thoughts and feelings).
The rule of mass is a scale. We know what something weighs by weighing it ie. Comparing it to an objective standard
The rule of length or width is a tape measure or ruler. We know how long something is by measuring it ie. Comparing it to an objective standard.
The rule of faith an objective standard to measure if what one believes is true.
The Primary Rule of Faith is God.
Since God chooses to communicate with us indirectly, we must use 2 Secondary Rules of Faith (that should always coincide).
The 2 Secondary Rules of Faith are:
(1) Proximate Rule of Faith (Magisterium)
(2) Remote Rule of Faith
(A) Sacred Scripture
(B) Sacred Tradition
I. Documents
a. Papal Encyclicals
b. Writings of the Saints
c. Other approved Sacred documents
II. Monuments
a. Approved Private Revelation
b. Sacramentals
c. Religious Artwork
d. Rituals/Liturgies
e. Churches
f. Other Sacred monuments
Remember, the Magisterium is the servant of Sacred Scripture and Sacred Tradition, not the inventor of it. The Magisterium may clarify teachings, but never contradict or invent them.
We are instructed to follow the steps of St Paul.
“I have passed on to you, first what I also received.”