This was not necessarily true. In fact, this is a huge generalization of facts. People realized that our planet wasn’t the center of the universe in 300 BCE.

Greek scientist, Aristarchus of Samos was the first person to suggest this idea. However, there was no way to prove this theory and only until the 16th century did astronomer Nicolaus Copernicus decide to work on this theory. There were a lot of people who disagreed with him but he also had a lot of supporters like for example, Galilei. So, Galilei did not suggest this theory but rather, he was the one to actually prove it thanks to the telescope he managed to invent.