Prior to the Christian era the cross was looked upon with disfavour.
To be crucified was to undergo a most ignominious form of punishment, and it was only served out to malefactors of the worst description. Nothing short of this would have been a sufficient check in those times to the growth of vice. But in the early days of Christianity the cross came to be regarded as the holiest symbol of "The Sacrifice" made for the good of mankind.
When converts met they formed on the ground the sign of the cross, in order to distinguish friends from foes. The mere fact of a severe punishment meted out consequent on discovery of this secret passport served only to increase the reverence held for the symbol.
As soon as time and opportunity allowed places of worship were erected, and the natural form adopted would be that of the cross, for which they had suffered so much persecution, and which typified the foundation of their faith and hopes of salvation.
As they assembled in church they would be sensible of the prevailing influence of the emblem. In every direction, look where they would, they would always see the holy sign. The roof would reveal to the gaze the same form as that on the ground.
Even the walls, as they soared upwards, out-lined, tier upon tier, the Christian sign, capped at the last by a mighty cross, which cast its protecting shadows around and over the worshippers.