Query Posts that Match All Categories?

Hi. I’ve searched and searched and haven’t been able to find the solution, though I know it must be simple.

I need to show a list of post titles that match all the categories of the current post. This is what I’ve got so far, but get_the_category( $id ) gets the ID of the first category, not all categories, thus the query returns all posts that match the first category. I need it to return only posts that match all categories.

<?php
					$match1 = get_post_meta($postid, 'classLevel', true);
					$match2 = get_the_category( $id );
					if($match1 = 'Beginner') {
					$args = array(
  						'post_type' => 'class',
  						'meta_key' => 'classLevel',
  						'meta_value' => 'Intermediate',
  						'category__in' => $cat_id,
  						'posts_per_page' => 20, //limit the post numbers,or use -1 for all
  						'post__not_in' => array($post->ID), //exclude current post
						);
					} else {
					$args = array(
  						'post_type' => 'class',
  						'meta_key' => 'classLevel',
  						'meta_value' => 'Advanced',
  						'category__in' => $cat_id,
  						'posts_per_page' => 20, //limit the post numbers,or use -1 for all
  						'post__not_in' => array($post->ID), //exclude current post
						);
					}
					$my_query = new WP_Query($args); ?><?php while ($my_query->have_posts()) : $my_query->the_post(); ?>
						<li><a href="<?php the_permalink(); ?>"><?php if (function_exists('get_cat_icon'))
						get_cat_icon('fit_width=12&fit_height=12&link=false&small=true&class=caticon'); ?><?php the_title();?></a></li>
					<?php endwhile;?>
					<?php wp_reset_query(); ?>

<?php echo get_the_category( $id ) ?> returns “array”–not an array, the word “array.”

How do I make the query return only those posts that match all the categories of the current post?

Thanks in advance for the help.