The Django Docs say that the AuthenticationForm is just that, it is a Form, so I believe that you would need to call it in this way:
from django.template import RequestContext
from django.contrib.auth.forms import AuthenticationForm
def main_page(request):
if request.method == 'POST':
form = AuthenticationForm(request.POST)
# log in user, etc...
else:
form = AuthenticationForm() # Unbound Form
return render(request, 'main_page.html', context_instance=RequestContext(request, {'form': AuthenticationForm}))