Whenever we replace Jesus with another lord, we displace the gospel from the center of our discipleship. We substitute Jesus's perfect performance for our imperfect performance, which will always fail. The gospel reminds us that our approval before God rests, not on our performance but on the performance of Jesus in his perfect life, death, and resurrection. Religious performance deceives us by saying: "Impress God and he will approve of you." The truth of the goepl, however, says: "You don't have to impress God because Jesus has impressed him for you." (71)
Substitution in the atonement as understood by evangelicals is a beautiful doctrine. But when the substitution you're practicing is one in which you substitute your good works in place of Christ's, to quote the younger generation, "you're doing it wrong."
Substitution in the proper sense is necessary precisely because our own good works are not sufficient to assuage God's wrath and make him pleased with us. Thus, Christ became sin for us and suffered the penalty due us, that we might be reconciled to God. Our sin was imputed to him; his righteousness was imputed to us.