"How do you know that the Council was wrong?"
Because God is just and there is no justice in what they concluded. A just God would not hold me stained with sins I did not commit.
What makes you think they were right? Because they were able to suppress all opposition to it long enough to make it a tradition that is still around today? That would be very poor reasoning. The age and popularity of a belief don't prove it's right.
What if you were really were stained by the sin of the first man, would you expect God to pretend that there was no such stain?
-A8