Why do people think the apostle Paul dominated 1st century Christian doctrine?

If the American Ambador wrote a book, claiming "this is what Americans believe, and you aren't an American unless you believe everything I wrote," and there were no more Americans alive for the French to ask (or any other text they could turn to), then it might be reasonable for those French people to look towards that ambador as an authority on Americans and the effective leader of the United States, even if he was being given orders by the President at the time that he wrote it.