The configuration file contains the encryption key that is used to encrypt your users passwords and other items such as the security questions. So if you copied the database, you will need the encryption key to correctly verify your users passwords, otherwise users will get wrong password even though they have entered the correct one.
Best bet, would be to restore your database and copy the encryption key from the configuration file over, otherwise, you'll just have to create new password for each customer using the new encryption key.