CF7: Custom Error Message

May I ask how is the wpcf7_form_response_output filter (found in includes/contact-form.php) use?

The below is what I had tried but to no avail.

Function 1:

$wpcf7->skip_mail = true;
$content = __('You have already subscribed to our Events.',CURRENT_THEME);
$class = 'wpcf7-validation-errors';
custom_wpcf7_form_response_output_filter('', $class, $content, '');

Function 2

if(!function_exists('custom_wpcf7_form_response_output_filter')){
    function custom_wpcf7_form_response_output_filter($output, $class, $content, $this){
        return '<div class="' . $class . ' ">' . $content . '</div>';
    }
    add_filter( 'wpcf7_form_response_output', 'custom_wpcf7_form_response_output_filter', 10, 4);
}

Any help here?