The cross represents a binary system: the horizontal limb corresponding to the passive principle or the world of phenomena and the vertical limb the active principle, that is, the transcendent world of spiritual evolution. 

Theosophically viewed, the Christian cross has, in two dimensions, vertical and horizontal lines with a point of intersection. The vertical pole is štma, or “spirit” and here descends the life of the Monad which is the highest aspect of the individual, thus forming the vertical shaft of the cross. The human constitution is made up of seven principles which are always present, no matter what the degree of development prevailing. Certain of these principles are active and some passive or subjective, the condition depending on the spiritual level attained. For the vast majority of humanity most of the principles are only partly awakened and at the particular level the condition is symbolized by the horizontal arm of the cross. Below the arm we find the spiritual levels that the individual has gained during the past lives.

The horizontal arm divides the objective “field” from the “subjective” in one of its functions, but may on the other hand, unite them; both are true since the horizontal is a moveable bridge.  Source