Quod in vetere latet in novo patet

For early Christians this brief incident [Moses and Joshua battling Amalek] became particularly significant, since it was a striking demonstration that, as the Latin saying had it, quod in vetere latet in novo patet (what is hidden in the Old [Testament] is made explicit in the New). As they saw it, Moses' hands in the form of the cross were only part of the hidden message. Surely it was not without significance that Moses himself stood to the side while his heretofore unmentioned assistant, Joshua, actually led the battle. The name Joshua in its Greek form is Jesus. Any early Christian who read the Bible in Greek simple knew that there were two biblical figures named Jesus, one in the Old Testament and one in the New. To the typologically minded, there could thus scarcely be any doubt that the Old Testament Jesus, taking Moses' place in the lead, was a foreshadowing of the one in the New whose religion would replace that of Moses - quod in vetere latet in novo patet. What is more, Amalek scarcely seemed like an ordinary human enemy. If he were, why should God, here and later on, single him out as Israel's archenemy [...] Amalek, Christians concluded, must symbolize the devil. This the fact that this Old Testament Jesus made his first appearance here, fighting the devil, while Moses stood to the side and made the sign of the cross, virtually clinched the case...

James Kugel, How to Read the Bible