Custom controls in the theme customizer using WP_Query() instead of query_posts(). Article on http://code.tutsplus.com/articles/custom-controls-in-the-theme-customizer

index.php

<div id="posts">
    <?php
    // Get category ID from Theme Customizer
    $catID = get_theme_mod( 'tcx_category' );
 
    // Only get Posts that are assigned to the given category ID
    $args = array(
        'post_type' => 'post',
        'cat' => $catID
    );
    $catquery = new WP_Query( $args );
 
    // The loop
    if ( $catquery->have_posts() ) {
        while ( $catquery->have_posts() ) {
            $catquery->the_post();
 
            the_title();
            the_excerpt();
            the_category();
            echo '<hr />';
        } // while
    } // if
    
    wp_reset_postdata();
    ?>
</div>