There are a lot of ways to look at the atonement and how it works. In Mere Christianity, C. S. Lewis speaks of different “theories” of the atonement and sort of brushes the differences under the rug– the point, he says, is that we are saved by Jesus’ death, not how we are saved by his death.
Many of the ways people speak about the atonement are absolutely true. Jesus did die as an example of self-sacrificing love. His death did disarm the powers of darkness. His suffering does show that he can identify with all all those who suffer unjustly in this world. But at the heart of the atonement is the idea of substitution: Jesus stands in our place, receiving the punishment we deserve from God for our sin. (This is actually what makes all the other views make sense, but that’s for another day.)
As an illustration of this point, take John the Baptist’s first words when Jesus first appears on the scene:
Behold, the Lamb of God, who takes away the sin of the world! John 1:29
The phrase “Lamb of God,” to all John’s Jewish hearers, would have brought to mind a very specific set of ideas: The lambs offered in the sacrificial system were a substitute offering. The priest would lay his hands on the animal, symbolically transferring guilt from God’s people to the lamb. Then the lamb was slaughtered, its blood poured out, to demonstrate God’s wrath against sin. The people were then counted innocent, because blood had been shed on their behalf. Substitution. Imputation of guilt, and therefore innocence. Propitiation (the removal of wrath). All these ideas are wrapped up in that little phrase.
Penal substitution is not the only thing there is to say about the atonement. But if it’s not central to what Jesus accomplished on the cross, then the title “Lamb of God” is meaningless.