filter a loop base on specific category

by

There’s no valid query argument – category, it could be category_name or cat. So for your query, you can use – 

$custom_cat_args = array(
    'posts_per_page' => 15,
    'order' => 'DESC',
    'orderby' => 'post_date',
    'cat' => get_cat_ID( 'سیاست' )
);

Or 

$custom_cat_args = array(
    'posts_per_page' => 15,
    'order' => 'DESC',
    'orderby' => 'post_date',
    'category_name' => 'سیاست'
);