loop not showing up when using a custom page template

Question

Okay So I want to create a Custom Page Template that displays my posts exactly the same as my index.php does. You can see the code of my index.php at the bottom.

So I create a .php file named customindex.php and use the exact same code of my index.php and the code on the top to mark it as custom page template:

<?php
/*
Template Name: Custom Index
*/

Problem: The page with the Custom Index page template won't display any posts and I don't know why.

Code of my index.php:

<?php get_header(); ?>
<h1 class="heading" style="width:100%"><?php _e( 'Beliebte Zitate', 'html5blank'); ?></h1>

<?php get_sidebar(); ?>

<section id="home">

    <?php get_template_part('loop'); ?>

</section>

    <div id="pagination">
        <?php html5wp_pagination(); ?><br>
    </div>

<?php get_footer(); ?>

Code of my customindex.php (nearly the same as index.php):

 <?php
/*
Template Name: Custom Index
*/ ?>
<?php get_header(); ?>
<h1 class="heading" style="width:100%"><?php _e( 'Beliebte Zitate', 'html5blank');   ?></h1>

 <?php get_sidebar(); ?>

<section id="home">

<?php get_template_part('loop'); ?>

</section>

<div id="pagination">
    <?php html5wp_pagination(); ?><br>
</div>

<?php get_footer(); ?>

My loop.php looks like this:

<?php

 if (have_posts()): while (have_posts()) : the_post(); ?>


<article id="post-<?php the_ID(); ?>" <?php post_class(''); ?>>
    LOTS OF STUFF
</article>

<?php endwhile;  ?>

<?php else: ?>

<article>
    <h2><?php _e( 'Sorry, nothing to display.', 'html5blank' ); ?></h2>
</article>

 <?php endif; ?>

Answer 1

You haven't created a query to return your post index results.

Let me back up...

In WordPress something called the "Main" query runs very early in the page load, and well before your template files load. That query retrieves the posts to display and also (more or less) determines which template file to use to display the results. That "Main" query retrieves different results based upon the URL requested, or you'd get exactly the same thing on all pages. The results on the index page are going to be different from the results on your custom page-- the results on your custom page is going to be the single page itself and not the index. That means that if you want an index on some secondary page, you need to create a query.

It could be as simple as:

$p = new WP_Query(array('post_type' => 'post'));
if ($p->have_posts()) {
  while ($p->have_posts()) {
    $p->the_post();
    // Display
  }
}

However, the peculiarities of your theme could complicate things, so be warned.