To authenticate (to check combination of username and password) you can use these methods:
Membership.ValidateUser(userName, password); // any provider
WAFMembershipProvider().ValidateUser(username, password); // directly on the WAF provider
WAFRuntime.Engine.AutenticateUser(...) // gives same result
user.Password.Test() // if you already got the user
To log in a user:
if (Membership.ValidateUser(userName, "password")) {
FormsAuthentication.SetAuthCookie(userName, true);
Response.Redirect("~/page.aspx");
}