I never said the Waldensians were first century. I suggest you read more carefully. Note I wrote, "As the Catholic Church grew in power and influence"--clearly post-first century.
"I suggest you read more carefully."
Sorry to disagree, but you should be a bit more careful with your writing.
You wrote “some of these groups--the Waldensians, for instance.” Here, the “these groups” refers back to the “apostolic groups [that] existed in the early centuries A.D” you used in the prior sentence. You then follow this by saying “It's really nonsense to claim that all first-century Christians were Catholic,” furthering the connection to the first century groups.
In addition to the history books, I would recommend "Handbook for Writers" by Lynn Quitman Troyka. (Simon & Schuster)
Simply wanting things to be true to fit your personal theology or writing style does not make them true.