httpcontext.current.user.identity.isauthenticated always false

Dear,
please help me, I create two pages, the first one is the login page with user name and password textboxes - (not asp login control) , when clicking login button I check the login authentication, if it is true redirect to default page. in the default page if !IsCallBack then i check httpcontext.current.user.identity.isauthenticated
if it is false i redirect the user to the login page. but my problem is that the httpcontext.current.user.identity.isauthenticated is always false.
Below the section of authentication in web config
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authentication mode Go to the complete details ...