How do I know the ‘meta_key’ for ordering my custom post type

What is “sermon date”, is it the custom field? If so, your meta_key is the name of your custom field used for that date. If not, how are you getting the sermon date?

Might be something like this:

$args = array(
    'post_type' => 'sermons',
    'meta_key'  => 'sermon_date',
    'orderby'   => 'meta_value',
    'meta_type' => 'DATE'
);
$query = new WP_Query( $args );