Printing first line of content in wordpress

What you can do is work with this the_excerpt() change this instead of the_content();

And give the amount of words that you want, add this in functions.php after you add the_excerpt();

function custom_excerpt_length( $length ) {
    return 20;
}
add_filter( 'excerpt_length', 'custom_excerpt_length', 999 );

Read more in here: https://developer.wordpress.org/reference/functions/the_excerpt/