A simple "Mazel Tov" or "Congratulations" is perfect to write in a Bar Mitzvah card, along with a personal note wishing the young man good luck in his adult life. A Bar Mitzvah is a Jewish religious ceremony which celebrates a young man turning 13. This is the age in which he becomes an adult member of the Jewish community.
For those of the non-Jewish religion, a Bar Mitzvah might be unfamiliar territory. In addition to it being a birthday party, it is also a significant day in a Jewish' boy's life. As with any birthday card, sentiments from the heart are the most welcome and meaningful. One should keep in mind, however, that this ceremony also signifies the transition from young boy into an adult. Therefore, sentiments in a Bar Mitzvah card should have a semblance of decorum appropriate to the occasion.
Bar Mitzvah wishes should celebrate and honor the birthday boy, as well as inspire him for the adult journey he is about to begin. The Torah is the book of Jewish law, and the basis upon which the religion was founded. Citing scripture from the Torah, or even referencing it in one's sentiment in the card, will be very meaningful to the recipient.