In Celtic times the cross was a protective symbol. A cross made of wood was particularly potent, for the wood itself was a protection.  On an archetypal level the cross symbolized the bridge to other worlds and to higher knowledge: the vertical axis represented the celestial world; and the horizontal axis represented the earthly world.  In some interpretations the arms represented the major elements of the universe:

 Earth-Air-Fire-Water, which are unified by Balance.