only displays certain category - php

Question

I'm building my own website just for fun and for learning.
I want to display only the post with a certain category # my homepage.
I've tried a number of things but they don't seem to work
at my home.php
this is what te code looks like.
get_header(); ?>
<?php $blog_layout = get_theme_mod('blog_layout', 'small-images'); ?>
<?php if ( ($blog_layout == 'masonry') || ($blog_layout == 'fullwidth') ) {
$layout = 'fullwidth';
} else {
$layout = '';
} ?>
<?php if ( $blog_layout == 'masonry' ) {
$masonry = 'home-masonry';
} else {
$masonry = '';
} ?>
<div id="primary" class="content-area <?php echo $layout; ?>">
<main id="main" class="site-main" role="main">
<span class="diensten">ONZE DIENSTEN..</span>
<div class="diensten-content">
<?php if ( have_posts() ) : ?>
<?php /* Start the Loop */ ?>
<div class="home-wrapper <?php echo $masonry; ?>">
<?php while ( have_posts() ) : the_post(); ?>
<?php
if ( $blog_layout == 'large-images' || $blog_layout == 'masonry' ) {
get_template_part( 'content', 'large' );
} else {
get_template_part( 'content', get_post_format() );
}
?>
<?php endwhile; ?>
</div>
<?php moesia_paging_nav(); ?>
<?php else : ?>
<?php get_template_part( 'content', 'none' ); ?>
<?php endif; ?>
</div><!--diensten-content-->
<
</main><!-- #main -->
</div><!-- #primary -->
And at content.php
<article id="post-<?php the_ID(); ?>" <?php post_class(''); ?>>
<?php if ( has_post_thumbnail() ) : ?>
<div class="entry-thumb">
<a href="<?php the_permalink(); ?>" class="H1-posts" title="<?php the_title(); ?>" >
<?php the_post_thumbnail('moesia-thumb'); ?>
</a>
</div>
<?php endif; ?>
<?php if (has_post_thumbnail()) : ?>
<?php $has_thumb = ""; ?>
<?php else : ?>
<?php $has_thumb = ""; ?>
<?php endif; ?>
<div class="post-content <?php echo $has_thumb; ?>">
<header class="entry-header">
<?php the_title( sprintf( '<h1 class="entry-title">', esc_url( get_permalink() ) ), '</h1>' ); ?>
<?php if ( 'post' == get_post_type() ) : ?>
<?php endif; ?>
</header><!-- .entry-header -->
<div class="entry-summary">
<?php if ( (get_theme_mod('full_content') == 1) && is_home() ) : ?>
<?php the_content(); ?>
<?php else : ?>
<?php the_excerpt(); ?>
<?php endif; ?>
</div><!-- .entry-content -->
I've tried this..`
<?php
//refine your query to the category you desire either a slug(example below) or category id
$args = array(
'category_name' => 'my_category_slug',
);
//create the query using the arguments
$query = new WP_Query($args);
?>
//create the loop to show the posts
<?php if($query->have_posts()): ?>
<?php while($query->have_posts()): $query->the_post(); ?>
<h1><?php the_title(); ?></h1>
<div><?php the_content(); ?></div>
<?php endwhile; ?>
<?php endif; ?>
did not work. How do I fix this?`If It displays only the posts with a certain class if also good.
Thanks..