And it came to pass, when Moses held up his hand, that Israel prevailed: and when he let down his hand, Amalek prevailed. But Moses' hands were heavy; and they took a stone, and put it under him, and he sat thereon; and Aaron and Hur stayed up his hands, the one on the one side, and the other on the other side; and his hands were steady until the going down of the sun. Exodus 17:11-12
"And again He speaks of this in Moses, when Israel was attacked by foreigners. And that He might remind those who were being attacked that they had been given over to death because of their sins—the Spirit speaks to the heart of Moses, that he should make a type of the cross, and of Him who was to suffer thereon. He is saying that if they do not place their hope on Him, they shall be under attack forever.
Moses therefore placed one shield upon another in the midst of the battle, and standing upon it, so as to be higher than all the people, he stretched forth his hands. And as long as he did so, Israel again prevailed; but whenever he let down his hands, they were again being killed. Why? So that they might know that they could not be saved unless they put their trust in him.
And again, in another prophet He says: "All day long I have stretched out my hands to a people who are disobedient and who oppose my righteous way (Isa 65:2)." And again Moses makes a type of Jesus—signifying that it was necessary for Him to suffer, and also that He whom they believed had perished would bestow life. For since transgression was committed by Eve through means of the serpent, the Lord brought it to pass that every kind of serpent bit them, and they were dying, so that He might demonstrate to them that it was because of their transgression they were given over to the straits of death.
Furthermore, it is this same Moses who commanded: "Ye shall not have any graven or molten image for your God (Deut 27:15)." He did so that he might reveal a type of Jesus. Moses then makes a brazen serpent and places it upon a beam, and by proclamation assembles the people. When, therefore, they were come together, they begged Moses that he would offer sacrifice in their behalf, and pray for their recovery.
And Moses spoke unto them, saying: "Whenever anyone of you is bitten, let him come to the serpent erected on the wooden pole; and let him hope, believing that even though dead, it is able to give him life, and immediately he shall be healed." And they did so."
(Ehrman, Bart D., The New Testament and Other Early Christian Writings, Oxford University Press, New York, 1998, “The Letter of Barnabas”, chapter 12:2-7, pgs 353-354)