One other thing about something specific said in this article. How could the selling of indulgences not have been grievously wrong, including according to Catholic belief. The Catholic Church teaches that there isn’t assurance of salvation, but unconfessed mortal sin puts one in Hell. Only God can know about all unconfessed sin. Selling indulgences presumes to know that a believer is saved, but only in Purgatory temporarily to be cleansed from their sins that weren’t mortal. That contradicts Catholic doctrine.
If the Great Apostasy had ever occurred in the past, there would be no belief in Jesus Christ today. The Great Apostasy is yet future.