how do we know?
the passing on of the Apostolic Faith from one generation to the next, that we know will occur until Jesus returns at the end of the age. contrary to what the false teachers that have arisen have put forth ( as Jesus predicted they would ) the elect can not be fooled.
Two thousand years of oral tradition and we’re supposed to swallow that it was passed on unchanged in that time?
What a joke.
God cares much more about maintaining orthodox truth than relying upon "word of mouth" to keep it pure. Mankind is NOTORIOUS for attaching legends, myths, boastings and biases into what actually IS already recorded in Holy Scripture (i.e.; the Apocryphal books) to leave it up to fallible men to keep it all straight orally. Hasn't it ever occurred to you how remarkable it is we have writings that stretch back THOUSANDS of years yet are still just as relevant and effective, touching and insightful, soul wrenching and comforting as they were when they first written? Why is that, you think? Could it possibly be because God is the author - the creator of all that is who knows the thoughts and intents of every heart that ever existed and ever will exist?
The "false" teachers that have popped up over the centuries either didn't bother with sacred Scripture or twisted what was written to fool those who did not take the time to know what it really said. These deceitful workers were disputed BY the Scriptures to prove their contentions WERE false. That is how I know when I hear a false doctrine being espoused - I know what the Word really does say and what it doesn't say. And, rather than depend upon another person's word that something is true or not, I do my own research which entails reading the word, studying past theologians, reading different translations of the verses in question including the Greek or Hebrew words, meditating upon the Word, prayer and listening to the Holy Spirit. That's why we are commended TO study in the first place. My eternal destiny is too precious a thing to entrust to sinful and fallible men. I go to the source and it is NOT a religion or organization of men. What was passed down from one generation to the next of the tenets of the Christian faith are found in the Bible and any group that purports to be the true faith had BETTER have the backing of Holy Scripture.