You can use the following action to redirect all traffic to that login page. You will need to specify the page name and the page slug.
add_action('wp', 'wpse_41699_wp', 0);
function wpse_41699_wp(){
if(is_page('Login'))
return;
if(!is_user_logged_in()){
$url = get_bloginfo('home').'/login';
header('HTTP/1.1 403 Forbidden');
header("Location: $url");
exit;
}
}
Also, you can download the plugin version for this answer here.