Hey man i had the same problem a couple of minutes ago.
It should work as follows:
<?php
if(have_posts()) {
/* Start the Loop */
query_posts('cat=51');
while (have_posts()) {
the_post();
get_template_part('content', 'post');
/* Display comments */
if ( theme_get_option('theme_allow_comments')) {
comments_template();
}
}
}
// go backwards to the original post for the custom menu
wp_reset_query();
if(have_posts()) : the_post(); endif;
?>