Illustrating the necessity of a substitutionary atonement, R. C. Sproul draws three circles. The first circle represents the character of man. The second represents the character of God. The third represents Christ.

Imagine a circle representing Jesus’ character. He lived as a man on earth for decades, subject to the Law of God and subject to all of the temptations known to man. (Heb. 4:15). But we do not see any blemishes in His circle. Not one. This is why . . . John the baptist cried, “Behold! The lamb of God who takes away the sin of the world!” (John 1:29b). The Passover lambs of the Old Testament were to be lambs without blemish, as physically perfect as possible. But the ultimate lamb, the Lamb of God who would take away the sins of His People, was to be perfect in every way. In calling Jesus the Lamb of God, John was affirming that Jesus was untouched by sin.
   Jesus Himself made this claim. He asked the Pharisees, “‘Which of you convicts me of sin?’” . . . How would you react if somebody said to you: “I am perfect. If you don’t agree with me, prove that I’m not.” That’s what Jesus said. He claimed to have no shadow of turning, no blemish, nom sin. He said that his meat and drink were to do the will of the Father. He was a man Whose passion in life was obedience to the Law of God.
   We have one unjust party (man) and two just parties. We have a just God, and a just Mediator, Who is altogether holy. The Mediator is the One who came to satisfy the requirements of a just God on behalf of the unjust race of man. He is the One who makes the unjust party just. He is the only One Who could do so.
. . . This justification takes place ultimately when the supreme Judge of heaven and earth says, “You are just.”
   The grounds for such a declaration are in the concept of imputation. . . . we are talking about imputation when we say that Jesus bore our sins, that He took the sins of the world on Himself. The language there is one of a quantitative act of transfer whereby the weight of guilt is taken from man and given to Christ. . . . In theological language, we say that God imputed those sins to Jesus.
   If all that happened was a single transfer of our sins to Jesus, we would not be justified. If Jesus took all the sins I’ve ever committed on His back and took the punishment for me, that would not get me into the kingdom of God. It would be good enough to keep me out of hell, but I would still not be just. I would be innocent, if you will, but still not just in the positive sense. I would have no righteousness . . .
   Thankfully, however, there is not just one transfer, there are two. Not only is the sin of man imputed to Christ, but the righteousness of Christ is transferred to us, to our account. As a result, in God’s sight the human circle is now both clean of all blemishes and adorned with glorious righteousness. Because of that, when God declares me just, He is not lying.

—R. C. Sproul, The Truth of the Cross (Reformation Trust, 2007), 91–95.