[6] Then Moses and Aaron went from the presence of the assembly to the entrance of the tent of meeting and fell on their faces. And the glory of the LORD appeared to them, [7] and the LORD spoke to Moses, saying, [8] “Take the staff, and assemble the congregation, you and Aaron your brother, and tell the rock before their eyes to yield its water. So you shall bring water out of the rock for them and give drink to the congregation and their cattle.” [9] And Moses took the staff from before the LORD, as he commanded him.
[Moses Strikes the Rock]
[10] Then Moses and Aaron gathered the assembly together before the rock, and he said to them, “Hear now, you rebels: shall we bring water for you out of this rock?” [11] And Moses lifted up his hand and struck the rock with his staff twice, and water came out abundantly, and the congregation drank, and their livestock. [12] And the LORD said to Moses and Aaron, “Because you did not believe in me, to uphold me as holy in the eyes of the people of Israel, therefore you shall not bring this assembly into the land that I have given them.” [13] These are the waters of Meribah, where the people of Israel quarreled with the LORD, and through them he showed himself holy.
(Numbers 20:6-13 ESV)
[8] And the LORD said to Moses, “Make a fiery serpent and set it on a pole, and everyone who is bitten, when he sees it, shall live.” [9] So Moses made a bronze serpent and set it on a pole. And if a serpent bit anyone, he would look at the bronze serpent and live.
(Numbers 21:8-9 ESV)
The promises of God and the deliverances of God always rested on belief. Unbelief always resulted in God's judgment. Moses strikes the rock twice because of Israel's rebellion and he did not believe the words of the Lord. The children of Israel were bitten by fiery serpents because they failed to believe that God was leading them to a better land.
However, God has alway provided a way of deliverance. It is through faith, or belief in what God says He is going to do. For the rebellious children of Israel, they were to acknowledge their wickedness and rebellion by looking to the bronze serpent to heal them.
Fast forward to the New Testament. I am a rebellious sinner who has a heart that is naturally inclined to evil. I distort boundaries so that I can accomplish my sinful agendas. I am ungrateful, unholy, and reject God in my actions and thoughts. God's judgment is upon me. But God provides a way of escape through belief in His Word, in his Son Jesus Christ. "And as Moses lifted up the serpent in the wilderness, so must the Son of Man be lifted up, that whoever believes in him may have eternal life." (John 3:14-15 ESV) I can be healed of my sin by looking to the Cross of Jesus Christ, where the Lord Himself bears my judgment and pays the penalty of my sin that I may be declared righteous. The Lord is Holy and He will not tolerate my sinful rebellion. The Lord Himself bears my judgment and provides a way of escape by believing in the good news of Jesus Christ.