Then you only need one Activity.
If a user is able to use your app in some capacity without logging in, then that screen / destination will be your root destination. On that screen presumably there’s a way for them to reach the login destination if they want to.
If a user must log in to make any use of your app, you can take the approach of having a splash/loading destination, which just shows a loading spinner for example, while your app checks if the user is logged in or not. If they’re logged in, go to your Main destination. Otherwise go to Login destination