Hey there buddy! So, you know how there are some people who believe in God and go to church every Sunday? Those people are called Christians. Well, in some parts of the world, like Europe and North America, not as many people are going to church anymore or believing in God like they used to. That's what we mean by the decline of Christianity in the Western world.
Now, there could be lots of reasons why this is happening. One reason might be that people are getting more interested in science and technology, and they want to find answers to the big questions of life in a different way. Another reason might be that people are more focused on things like work, school, or entertainment, and they don't have as much time or interest in going to church.
Whatever the reasons, it's important to remember that people are allowed to believe or not believe whatever they want. And just because fewer people are going to church doesn't mean that Christianity is going away completely. There are still lots of people who find comfort and joy in their faith, and that's okay too!