From this article, False Brethren and False Principles in the Church: Spirit and Character of the Christians:

It is a curious fact that the figure of the instrument of torture on which our Lord was put to death, occupied a prominent place among the symbols of the ancient heathen worship. From the most remote antiquity the cross was venerated in Egypt and Syria; it was held in equal honour by the Buddhists of the East, [316:2] and, what is still more extraordinary, when the Spaniards first visited America, the well-known sign was found among the objects of worship in the idol temples of Anahuac. [316:3] It is also remarkable that, about the commencement of our era, the pagans were wont to make the sign of a cross upon the forehead in the celebration of some of their sacred mysteries. [317:1] A satisfactory explanation of the origin of such peculiarities in the ritual of idolatry can now scarcely be expected; but it certainly need not excite surprise if the early Christians were impressed by them, and if they viewed them as so many unintentional testimonies to the truth of their religion. The disciples displayed, indeed, no little ingenuity in their attempts to discover the figure of a cross in almost every object around them. They could recognise it in the trees and the flowers, in the fishes and the fowls, in the sails of a ship and the structure of the human body; [317:2] and if they borrowed from their heathen neighbours the custom of making a cross upon the forehead, they would of course be ready to maintain that they thus only redeemed the holy sign from profanation. Some of them were, perhaps, prepared, on prudential grounds, to plead for its introduction. Heathenism was, to a considerable extent, a religion of bowings and genuflexions; its votaries were, ever and anon, attending to some little rite or form; and, because of the multitude of these diminutive acts of outward devotion, its ceremonial was at once frivolous and burdensome. When the pagan passed into the Church, he, no doubt, often felt, for a time, the awkwardness of the change; and was frequently on the point of repeating, as it were automatically, the gestures of his old superstition. It may, therefore, have been deemed expedient to supersede more objectionable forms by something of a Christian complexion; and the use of the sign of the cross here probably presented itself as an observance equally familiar and convenient. [318:1] But the disciples would have acted more wisely had they boldly discarded all the puerilities of paganism; for credulity soon began to ascribe supernatural virtue to this vestige of the repudiated worship. As early as the beginning of the third century, it was believed to operate like a charm; and it was accordingly employed on almost all occasions by many of the Christians. “In all our travels and movements,” says a writer of this period, “as often as we come in or go out, when we put on our clothes or our shoes, when we enter the bath or sit down at table, when we light our candles, when we go to bed, or recline upon a couch, or whatever may be our employment, we mark our forehead with the sign of the cross.” [318:2]