The vatican is a museum with some but not all art of religious themes. You pay to tour the museum entering St peter’s is free. Inside the basilica, they can provide a few interesting tidbits such as “this Pope authorized this” or “That Pope commissioned that” but you’re most left free to roan about the basilica. Inside the museum, you need an art expert though as it IS an art museum. It is a very special and unique museum but it is an art museum.
I should mention that all kinds of people not just catholics can enjoy the museum and its treasures.