Force SSL on WordPress Pages, in this case, a page with an ID of 2

gistfile1.php

<?php

$using_ssl = isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] == 'on' || $_SERVER['SERVER_PORT'] == 443;
add_action('wp', 'check_ssl');
function check_ssl()
{
    // Page ID 2 must be https
    if (is_page(2) && !$using_ssl)
    {
        header('HTTP/1.1 301 Moved Permanently');
        header('Location: https://' . $_SERVER['SERVER_NAME'].$_SERVER['REQUEST_URI']);
        exit;
    }
    // All other pages must not be https
    else if (!is_page(2) && $using_ssl)
    {
        header('Location: http://' . $_SERVER['SERVER_NAME'].$_SERVER['REQUEST_URI']);
        exit;
    }
}

?>

You may want to add a variable for the page ID: https://gist.github.com/seangates/23290bd3946cbe5c2a0e

Otherwise you may forget to change it in every place it's needed.