The cross is one of the simplest geometrical figures: a horizontal line and a vertical line intersect at right angles. But let’s study the two directions. The horizontal direction is one of spreading and dispersion, like water spreading out over a surface. In contrast, the vertical direction is one of unification, like fire leaping up into the sky.
There is something about fire’s form that recalls the shape of a mountain with its base and summit. So the horizontal line is that of matter, the vertical line that of spirit. And these two lines are not separate; they meet, or, to be precise, they ‘cross’, and this shows not only that the two directions are not incompatible but also that they have something to do together. The symbol of the cross invites us, therefore, to continue to carry out our work on matter and, at the same time, take the vertical direction, in order to return to the spirit, the source, the summit.

A true understanding of the cross requires that you understand the nature and work of these two principles represented by fire and water, otherwise you will throw water onto your fire – symbolically speaking – and put it out; or you will plunge fire into water and the water will evaporate.

The cross, therefore, represents the masculine and feminine principles at work together in the universe. But their activity stems from a centre: the intersection of the two branches of the cross. This centre unites the forces and holds them together; without it, as soon as the cross began to revolve, everything would be scattered over the surface of the disc…

Omraam Mikhael Aivanhov
Izvor Book 218, The Symbolic Language of Geometric Figures
http://www.prosveta.co.uk/flip-through/P0218AN