the_content of template page

Question

When I use the_content in the page-template doesn't show anything.

Loop to call the page-template in the index:

index.php

<?php
/**
 * Main template file
 *
 * This is the most generic template file in a WordPress theme
 * and one of the two required files for a theme (the other being style.css).
 * It is used to display a page when nothing more specific matches a query.
 * E.g., it puts together the home page when no home.php file exists.
 * Learn more: http://codex.wordpress.org/Template_Hierarchy
 *
 * @package WordPress
 * @subpackage Twenty_Eleven
 */

get_header();
$pages = get_pages();
foreach ($pages as $page_data) {
    $title = $page_data->post_title;
    $template =  get_post_meta( $page_data->ID, '_wp_page_template', true );
    switch($template) {
        case 'template-portfolio.php':
        get_template_part( 'includes/portfolio' );
        break;
        case 'template-landing.php':
        echo get_template_part( 'includes/landing' );
        break;
    }

}

get_footer();

landing.php:

<div id="opening" style="width:100%; height:100vh;  background: white; background-size:cover;  margin: 0 0 -86px 0; ">
        <div id="text_opening">
    <!--<?php  $paged = (get_query_var('paged')) ? get_query_var('paged') : 1;  ?>
    Stai visitando la pagina <?php echo $paged; ?>-->

    <?php while ( have_posts() ) : the_post(); ?> <!--Because the_content() works only inside a WP Loop -->
        <div class="entry-content-page">
            <?php the_content(); ?> <!-- Page Content -->
        </div><!-- .entry-content-page -->

    <?php
    endwhile; //resetting the page loop
    wp_reset_query(); //resetting the page query
    ?>
    </div>
</div>

template-landing.php :

<?php
/*
Template Name: Landing
*/
get_header();
echo get_template_part( 'includes/landing' );
get_footer();

But it doesn't work, i can see only the div with the class "entry-content-page" but it's empty
I need to display only the content.
The strange thing is that if I click on "view page" in the admin panel, the preview show the content but in my site isn't there.
Any ideas?
Thanks!

EDIT:

PAGE VIA "VIEWPAGE": PAGE IN THE HOME:

EDIT2:

I tried to go to the "reading setting" and if I set that page (template-landing) like front page I can see the content

Answer 1

Have you tried using the whole loop?

<?php $loop = new WP_Query();
if ( $loop->have_posts() ) : 
    while ( $loop->have_posts() ) : 
    $loop->the_post(); ?>
    <!-- before -->
        <?php the_content(); ?>
    <!-- after -->
    <?php endwhile; else: ?>
    <p><?php _e( 'Nope. Doesn\'t exist' ); ?></p>
<?php endif; ?>

Answer 2

SOLVED IN THIS WAY: (index.php)

<?php 
             
            $args = array(
            'child_of' => 0,
            'order' => 'ASC',
            'orderby' => 'menu_order',
            'hierarchical' => 1,
            'post_type' => 'page',
            'post_status' => 'publish',
            'post_parent' => '0',
            'posts_per_page' => '100'
            ); 

            

      $wp_query = query_posts($args);
      query_posts($args); // Query all published pages without parents
      $first_loop = true;
              
        if (have_posts()) :
        while (have_posts()) : the_post(); 
            //Determine template for page style
                        $template = get_post_meta( get_the_ID(), '_wp_page_template', true );
                        $parent_id = get_the_ID();
                        
                         switch($template) {
                             
                            case 'template-portfolio.php':
                                    get_template_part( 'includes/portfolio' );                                                      
                            break;
                        case 'template-landing.php':
                                    echo get_template_part( 'includes/landing' );                                                   
                        break;                       
                             
                         }
        endwhile;
        endif;
        ?>