The Sacred Monogram known as Labarum. Both name and emblem were very possibly adapted from the primitive cult of the Labrys, or Double Axe, filtered through Mithraism. The figure is never found as a Christian emblem before Constantine, though it appears as a Heathen symbol upon the coinage of Decius (A.D. 250). See Parsons, 'Non-Christian Cross,' p. 148.
Hilary (A.D. 358), 'De Synodis,' § 2.
Ammianus Marcellinus, 'Hist.' XX. I.