How to implement ServerCertificateValidationCallback in ASP.NET MVC?

Question

I would like to add client authentication through X509 certificates, so I installed my IIS server with SSL certificate and client . (as said here: Client authentication through X509 certificates in asp.net )

But as soon as the client selects a certificate, the server always responds with HTTP 403.

Therefore, in order to configure client certificate verification, I added to global.asax:

     protected void Application_Start()
     {
        .......
        ServicePointManager.ServerCertificateValidationCallback += new RemoteCertificateValidationCallback(ValidateServerCertificate);

        private static bool ValidateServerCertificate(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
        {            
         //... checking
         return true;
        }
     }

But apparently, my function is never called, and the server response is still allowed 403 (the certificate is considered unsafe).

Do you have any suggestions?