As we read in Numbers today we remember the sin that Aaron and Moses committed against God's command at Meribah. Though the Lord sent them on about the work he had appointed to them he did not forget their sin. Aaron, at the end of his life, was stripped of his priesthood, his office, and his opportunity to enter the promised land. We see in this incident, and in the incident from Numbers 21.4-9, that the Lord does in fact bring justice against sin. Those who doubt and despise his commands will perish.
In the plague of the serpents, see how the Lord both administers justice and mercy. He has one like the deadly one raised up for everyone to see. Yet looking upon this serpent brings life. Rather than biting and bringing death, this serpent who is lifted up brings life. How does it accomplish this? Not by any apparent activity, but by the belief of the person who trusts God's word. Our Lord has said that to look upon this deadly creature lifted up in public people will live. So they do. This is not by any power or righteousness of their own, but only by God's word and promise.
Likewise we see in the New Testament that Christ, lifted up for us, raised up on a pole, the one like us except in our sin, the one who has become sin for us, this Jesus brings life. He does not bring life because of our righteousness. It is due to no good work on our behalf, but due to his becoming sin, becoming death, for us. How do we receive this? We receive it just like the Israelites did. We receive life by believing that what God has said about the Son is true. We receive life by trusting that Jesus gives us life.
Thanks be to our Lord, who has taken the sin of the world on himself and has himself been raised up in death on our behalf, so we may look upon him and live.