When you visit Rome, you are very lucky! Because you can literally walk to another country... THE VATICAN.
This is where the pope lives, the head of the catholic church.
Since the Vatican and St. Peter's Basilica are so immensely large, it is highly recommended to visit them with a guide. Although you will have to join a typical tourist group following a little flag for a few hours, it is definitely worth it to learn more about the architecture and history behind the Vatican.