Andreas Osiander was a man who lived a long time ago in a place called Germany. He was a really smart man who studied and wrote about religion. He wrote a book called "Mysterium Magnum" which means "The Great Mystery" in English. That book was about God and how people should follow his teachings.
One thing that made Andreas Osiander a little bit different was that he believed in something called "justification by faith". This means that people can be forgiven for their sins and go to heaven just by believing in God and having faith, without having to do any special things like fasting or giving money to the church.
Some people didn't like this idea and thought it was wrong. They argued with him and said that people needed to do more to be saved. But Andreas Osiander didn't change his mind, and he kept writing and teaching about his beliefs.
Later on, some people started calling Andreas Osiander a heretic, which means someone who has different beliefs than most other people in their religion. But even though people didn't always agree with him, Andreas Osiander was still an important person in the history of religion and ideas.