_init,php isn't a regular template file.
It's more kind of a universal settings file with variables and presets you might need later on your page(s).
So you could add the following line to site/config.php
$config->prependTemplateFile = '_init.php';
The _init.php in /site/templates/ will be included every page load and therefore the login check is always in place.
Within this file you could and should add psy's / your code
<?php
$loginPage = $pages->get('template=loginregister'); // in my case yours might be different
if(!$user->isLoggedin() && $page->id!=$loginPage->id) { // checks if user is logged in and not on login page
$session->set('returnPage', '/path/to/welcome/'); // set your welcome site
$session->redirect($loginPage->httpUrl);
die;
};
This would redirect every user that is not logged in to your login page and sets the returnPage.
Be careful with the check conditions. You might want to fine tune this check.