When Yosef reveals himself to his brothers he says, “Your eyes can see and the eyes of my brother Binyamin that it is my mouth that is talking to you.” (Gen. 45:12) Rashi explains that he meant they could hear him speaking in Hebrew and not in the Egyptian language. Isn’t it strange that he says your eyes can see and not you ears can hear?
Someone once suggested the answer that Yosef not only depended on his speaking to them but he also showed them his bookcases, that he had Jewish scrolls and only items of Jewish interest. By what he collected and what he considered important he showed how Jewish he was.
Today also, it is very important for Jewish people to have Jewish books and Jewish art works to demonstrate that the home is Jewish. When I would speak to teenagers I would always ask them in what way was their house different from their non-Jewish neighbors. If they told me they had a Bible or a Siddur or other Jewish books in the house I knew there was some connection in that house with Judaism. If they had nothing it was a good sign that they were far removed from Judaism and from ties with the Jewish people.