Why? It's a big religion in the world and I heard it grows at 30% per year

typo? Rounding it up to 2 billion, 30% means 600 million per year

How much of that is just because people aren't allowed to leave the religion though? My whole family would be considered Catholic if we still had those sorts of old thinking rules that Islam still has. Instead we have lots of people becoming Catholic and lots leaving balancing out.