The triangle refers to the supernal triad formed by the Kether, Chokmah Binah spheres but that does not explain the Red Cross.  So lets look at something else. The tetractys was a triangle symbol which was considered holy by the Pythagoreans.  It showed the whole of created matter as a series of ten dots. The first dot represented the One Thing, the Second the Duality, the Third the Mediating World Soul and the fourth the four elements of the material word.    Now the central dot was the most important as it represented the mediating influence of the World Soul from the upper to the lower words.  At the time of the Chaldean Oracles this role belonged to Hekate, but was similar to