WP_ForceLogin.php

<?php

// functions.php 
function forceLogin(){
    if(!is_user_logged_in()){
        $redirect = TRUE;

        foreach(['wp-admin', 'wp-login'] as $url) {
            if(strpos($_SERVER['REQUEST_URI'], $url) !== FALSE){
                $redirect = FALSE;
            }
        }

        if($redirect){
            auth_redirect();
        }
    }
}

if(defined('DEV') && DEV === TRUE){
    forceLogin();
}