The Authenticate method works against a list of users and passwords stored in web.config.
So to use this, your web.config needs to look something like:
<authentication mode="Forms">
<forms name="LoggedInUser" loginUrl="~/Login.aspx" protection="All" timeout="10" path="/" />
<credentials passwordFormat="SHA1">
<user name="user1" password="27CE4CA7FBF00685AF2F617E3F5BBCAFF7B7403C" />
<user name="user2" password="D108F80936F78DFDD333141EBC985B0233A30C7A" />
<user name="user3" password="7BDB09781A3F23885CD43177C0508B375CB1B7E9"/>
</credentials>
</forms>
</authentication>
This example was obtained from the Microsoft page describing the authenticate method.
Also, it is important to note that Microsoft has declared this method obsolete and suggests using one of the Membership providers instead.