2 Samuel 22:36
Well, Miriam, IMHO the Star of David symbolizes exactly this idea: that the Lord is Israel's shield and not one's own strength. King David defeated Goliath BECAUSE the Lord was his Shield. The struggle between the belief in physical and spiritual power is expressed in the clearest way in 1Sa 17:45:
"Then said David to the Philistine, Thou comest to me with a sword, and with a spear, and with a shield: but I come to thee in the name of the LORD of hosts, the God of the armies of Israel, whom thou hast defied".
In the Star of David one triangle point up while the other point to the opposite direction. The physical and the spiritual are opposites that live together in the same shape which symbolizes the unity of the opposites.
Pay attention to the fact that most of the verses come from Psalms which, according to the Jewish tradition, were written by King David.