How can I retrieve the value of variable I have defined in a config form from a Twig template?
The values saved as configuration items can be accessed using code similar to the following one.
$slogan = \Drupal::config('system.site')-&gt;get('slogan');

Since you are asking for a template file, that code should be split in two parts. My example is for the node.html.twig template file, but you can use the same kind of code for other template files too.

In the preprocess function of your template file (e.g. mytheme_preprocess_node(&amp;$variables)) you get the value of the configuration item
$variables['slogan'] = \Drupal::config('system.site')-&gt;get('slogan');


In the template file (e.g. node.html.twig), you access that variable
{{ slogan }}



Be sure not to use a variable ($variables['slogan'], in my example) that is not already used from the template file, or its preprocess functions.