Tertullian

But, to come now to Moses, why, I wonder, did he merely at the time when Joshua was battling against Amalek, pray sitting with hands expanded, when, in circumstances so critical, he ought rather, surely, to have commended his prayer by knees bended, and hands beating his breast, and a face prostrate on the ground; except it was that there, where the name of the Lord Jesus was the theme of speech-destined as He was to enter the lists one day singly against the devil-the figure of the cross was also necessary, (that figure) through which Jesus was to win the victory? Why, again, did the same Moses, after the prohibition of any "likeness of anything," set forth a brazen serpent, placed on a "tree," in a hanging posture, for a spectacle of healing to Israel, at the time when, after their idolatry, they were suffering extermination by serpents, except that in this case he was exhibiting the Lord's cross on which the "serpent" the devil was "made a show of," and, for every one hurt by such snakes-that is, his angels -on turning intently from the peccancy of sins to the sacraments of Christ's cross, salvation was outwrought? For he who then gazed upon that (cross) was freed from the bite of the serpents.