According to some Kabbalah sources the Star/Shield of David signifies the number 7: that is the six points plus the center. The earliest Jewish text to mention this is the "Eshkol Ha-Kofer"by a Karaite named Judah Hadassi, from the 12th century CE: "Seven names of angles precede the mezuzah: Michael, Gabriel, Nuriel, etc... protect you! And likewise the sign, called the "shield of David", is placed beside the name of each angel".

What is the significance of the number 7 in Judaism?

The number 7 has a special significance: the six days of creation + the seventh day of rest, the six working days of the week + Shabbath, the Seven Spirits of God, as well as the Menorah in the ancient temple, whose seven oil lampsrest on three stems, branching from each side of a central pole. The Magen David came to be used as a standart symbol because its organization into 3+3+1 corresponds to the more traditional symbol for Judaism in ancient times. See Star of David Jewelry