Filter out posts from index page to show only one specific category

functions.php

function customize_query ( $query ) {

  // Make sure this is only effecting the public side
	if ( ! is_admin() ) :
    // Make sure this is a query for the post page and is the main query for the page
		if ( is_home() && $query->is_main_query() ) :
      // Set query to only pull posts from the specified category
			$query->set( 'category_name', 'Newsletter' );
		endif;
	endif;

}

add_action( 'pre_get_posts', 'customize_query' );