A child theme is a theme that inherits the functionality and styling of another theme, called the parent theme. Child themes are the recommended way of modifying an existing theme. (This explanation is from the WordPress Codex, which contains information about WordPress in general and child theme’s in specific.) If you only want to change CSS styles, there’s no need to use a child theme; just use the Add CSS option in the Customizer (if you have JetPack turned on). Use this child theme if you want to make functionality changes.
You can download the starter child theme and customize it for your congregation.