Ishida refers to 3 different people. If you are talking about Uryū Ishida he didn't know his father was a Quincy. When he was a child his father told him there was no reason for him to be a Quincy because there was no profit in doing so. Therefore Uryū assumed that his father was not a Quincy and gave himself the title of the "Last Quincy". Later it was revealed the his father Ryūken was actually indeed a Quincy.