ex cathedra[/i]]For the Holy Spirit was promised to the successors of Peter not so that they might, by his revelation, make known some new doctrine, but that, by his assistance, they might religiously guard and faithfully expound the revelation or deposit of faith transmitted by the apostles.
This is the dogma that Myrna was talking about. God has revealed what He has revealed, and therefore there can be no new revelations from which to draw new doctrine. But, in many questions He did so in a general fashion, since the Apostles and the world were as yet unable to bear them in their fullness, hence we need a Vicar of Christ on earth to settle disputes as to the true meaning of revelation. He therefore ordained that by the Holy Ghost it would be St. Peter and his successors who should "teach all truth" (St. John 16:12-13 , 21:15-18), and "bring all things to your mind, whatsoever I shall have said to you" (St. John 14:26).
How do we know 'what the Church teaches',
By believing first those things which are infallible and measuring all other things against that, rejecting what is contrary (and those who offer it) and retaining what conforms. (Galatians 1:8-9)