Before an user can login, an account needs to be created.
An important aspect of creating an account is that the user agrees with the terms of service. That is why all accounts need to be created explicitly and it is not possible to create a new account on the fly when the user logs in using an external identity provider for the first time.
When the user creates a new account using their e-mail address and a password, the e-mail address needs to be validated first. For this the user receives an activation e-mail with a link the user most follow.