The CategoryTree is a MediaWiki extension that provides a dynamic view: you can change how much you see of a category's structure as a tree. You can use it to show a category and its subcategories by putting the category name between category tree tags like this:
<categorytree> Trees in religion </categorytree>

The result is a category tree like the one shown to the right. Click on the triangles to expand and collapse the tree.

To add this auto-updating template to your user page, use {{totd b}}