Then Moses answered, “But behold, they will not believe me or listen to my voice, for they will say, ‘The Lord did not appear to you.’” The Lord said to him, “What is that in your hand?” He said, “A staff.” And he said, “Throw it on the ground.” So he threw it on the ground, and it became a serpent, and Moses ran from it. But the Lord said to Moses, “Put out your hand and catch it by the tail”—so he put out his hand and caught it, and it became a staff in his hand— “that they may believe that the Lord, the God of their fathers, the God of Abraham, the God of Isaac, and the God of Jacob, has appeared to you.” Again, the Lord said to him, “Put your hand inside your cloak.” And he put his hand inside his cloak, and when he took it out, behold, his hand was leprous like snow. Then God said, “Put your hand back inside your cloak.” So he put his hand back inside his cloak, and when he took it out, behold, it was restored like the rest of his flesh. “If they will not believe you,” God said, “or listen to the first sign, they may believe the latter sign. If they will not believe even these two signs or listen to your voice, you shall take some water from the Nile and pour it on the dry ground, and the water that you shall take from the Nile will become blood on the dry ground.” (Ex 4:1-9)
I have mentioned in a previous post how the sign of "the serpent" is probably not the first one we would think of as a sign from God. Moses himself was afraid and ran from it. Sickness too is not something we would consider as a sign from God. Some people still struggle to come to terms with God's sovereignty over and in sickness, but that's another matter... What is important here is that the signs that God gave to Moses was not just that he could produce a serpent and sickness, but that he had authority over them.
Power over sickness and Satan were the same signs that Jesus himself used to prove that he had authority:
For which is easier, to say, ‘Your sins are forgiven,’ or to say, ‘Rise and walk’? But that you may know that the Son of Man has authority on earth to forgive sins”—he then said to the paralytic—“Rise, pick up your bed and go home.” And he rose and went home. When the crowds saw it, they were afraid, and they glorified God, who had given such authority to men. (Mt 9:5-8)
But Jesus rebuked him, saying, “Be silent, and come out of him!” And the unclean spirit, convulsing him and crying out with a loud voice, came out of him. And they were all amazed, so that they questioned among themselves, saying, “What is this? A new teaching with authority! He commands even the unclean spirits, and they obey him.” (Mk 1:25-27)
It is this very authority, that Jesus delegated to his first disciples:
And he called to him his twelve disciples and gave them authority over unclean spirits, to cast them out, and to heal every disease and every affliction. (Mt 10:1)
...but more than this, we are told in the Scriptures that these signs of authority shall follow all who believe.
And these signs will accompany those who believe: in my name they will cast out demons; they will speak in new tongues; they will pick up serpents with their hands; and if they drink any deadly poison, it will not hurt them; they will lay their hands on the sick, and they will recover.”
Praise God that he has given us his authority over Satan and sickness. We do not have to fear them, nor run from them, but reach out and lay our hands on them, and in the authority of Christ command them to submit.
But there is still a greater sign....
The seventy-two returned with joy, saying, “Lord, even the demons are subject to us in your name!” And he said to them, “I saw Satan fall like lightning from heaven. Behold, I have given you authority to tread on serpents and scorpions, and over all the power of the enemy, and nothing shall hurt you. Nevertheless, do not rejoice in this, that the spirits are subject to you, but rejoice that your names are written in heaven.” (Lk 10:17-20)
The greatest sign of all is that of blood and water. The sign that flowed out of the Saviour's pierced side. The sign that declares we are purchased for God, and that our names are written in the Lamb's book of life.