If you wanted to do it purely in Flash, you could set a global variable.
public class GlobalData
{
public static var authenticated:Boolean;
}
In Main.as...
GlobalData.authenticated = false;
In your login page, on successful login:
GlobalData.authenticated = true;
In Index page...
Gaia.api.beforeTransitionIn(checkAuthentication);
private function checkAuthentication(event:GaiaEvent):void
{
if (!GlobalData.authenticated) Gaia.api.goto(Pages.LOGIN);
}
In site.xml, set indexFirst to true.
<site indexFirst="true">