Conditionally display content depending on whether the user is logged in or not

gistfile1.php

<?php

// https://www.facebook.com/groups/wordpress.support/permalink/497037907031944

function mc_497037907031944( $content ) {
	if ( is_user_logged_in() )
		return $content;
	else
		return 'Sorry, you must be logged in...';
}
add_filter( 'the_content', 'mc_497037907031944' );