Because Jesus told the fledgling Church that He would send the Holy Spirit to guide them and that He would be with them until the end of world. We believe in His word. Notice that even Paul uses, over and over, terminology that says that the teaches whether oral or written by the Church is to be believed and followed over any man's interpretation.
>> “Notice that even Paul uses, over and over, terminology that says that the teaches whether oral or written by the Church is to be believed and followed over any man’s interpretation.” <<
Slippery falsehood!
Paul told his flock to hold what he said and what he wrote, and both of those are the same, exactly to the word.
Paul was not a deceiver, and would never speak that which he didn’t put in writing for the future. Paul was especially aware of the coming falling away and corruption, and the absolute requirement for all to be preserved in the written word, as it was, and spoke to it by telling each congregation to share the letters they received with all the other congregations.
Paul also knew that the church was only a mystic body, and never spoke nor wrote otherwise.
The Lord has called us out of Churchianity, calling it the Harlot and her daughters.