Try this
<?php global $post; ?>
<article id="post-<?php the_ID(); ?>" <?php post_class( 'entry' ); ?><?php echo et_fable_get_background(); ?>>
<div class="container clearfix">
<header class="entry-title">
<?php if ( is_single() ) : ?>
<h1><?php the_title(); ?></h1>
<?php else : ?>
<h2>
<a href="https://wordpress.stackexchange.com/questions/208008/<?php the_permalink(); ?>"><?php the_title(); ?></a>
</h2>
<?php endif; ?>
<?php //et_fable_post_meta(); ?>
</header>
<?php if ( ! is_single() ) : ?>
<div class="entry-content clearfix">
<p><?php truncate_post( 900 ); ?></p>
</div> <!-- .entry-content -->
<?php //This is page id or post id
$content_post = get_post($post->ID);
echo $content = $content_post->post_content;
?>
<footer class="entry-footer">
<a href="https://wordpress.stackexchange.com/questions/208008/<?php the_permalink(); ?>" class="read-more"><span><?php esc_html_e( 'Read More', 'Fable' ); ?></span></a>
</footer>
<?php endif; ?>
</div> <!-- .container -->
<?php if ( is_single() ) : ?>
<?php get_template_part( 'includes/share', get_post_format() ); ?>
<?php endif; ?>
<?php echo apply_filters('the_content', $post->post_content); ?>
</article> <!-- .entry-->