Exodus 4:3
And he said, Cast it on the ground. And he cast it on the ground, and it became a serpent; and Moses fled from before it. And he said, Cast it on the ground. And he cast it on the ground, and it became a serpent; and Moses fled from before it.
In the land of Midian Moses pleaded for a sign confirming that his message was from heaven, in a setting like those of Gideon and Elisha where a sign confirmed a heavenly message. The sign answered his fear that Israel would not believe God had sent him, would not listen to his voice, but would say the LORD had not appeared to him, by giving evidence appealing to his own senses. At the command he cast his rod on the ground, it became a serpent, he fled from it in fear, and when taken up it became a rod again.
Joined to the sign of the leprous hand, it assured him that both his own people and Pharaoh would be convinced that One mightier than the king of Egypt was at work among them. It assured him that God would work by such signs and wonderful works to make Pharaoh and the Egyptians fear so they would not dare to harm him, and to convince the king and his people that One greater than Moses was manifesting power before them. Even after many such miracles performed before Pharaoh in the sight of the people, they still would not let Israel go.
The serpent was probably a poisonous cobra, recognized by Moses' flight and by the word nahash associated with hissing, and with its swollen neck it represented royal and divine power on Pharaoh's crown, so the change was both a portent and a pledge of victory over Egypt's king and gods. Israel's deliverance was to be accomplished in a manner to pour contempt on human pride, with the deliverer going forth as a humble shepherd with only a rod in his hand, which God made the symbol of His power. The simple staff shows that God can do a great work through humble, simple, or surprising means devoted to Him, so His power and choice of agencies must not be limited.
The shepherd's crook became the rod of authority for leading more than a million idolatrous, rebellious people after leaving the flock of sheep, while Moses himself was to be a willing instrument worked by Christ, depending on the invisible Leader. Prepared in the school of shepherd work in humility, faith and trust, and united in close communion with the chief Shepherd, he became the shepherd of God's own people. Through his firm faith and abiding trust blessings reached Israel as God worked through him for their uplifting and salvation.