R. C. Sproul draws three circles illustrating the separation between God and man that made a substitutionary atonement by a God-man necessary. The first circle represented the character of man. Sproul continues:
Imagine a second circle, just like the one we had for man, to represent the character of God. How many blemishes would we see in this circle? Absolutely none. We are totally depraved, but God is absolutely holy. In fact, He is too holy to even look at iniquity. He is perfectly just.
Here, then, is the crux of the problem: how can an unjust person stand in the presence of God? Or, to put the question another way, how can an unjust person be made just, or justified? Can he start all over again? No. Once a person commits one sin, it is impossible for him ever to be perfect, because he’s lost his perfection with his initial sin. Can he pay the penalty for his sin? No—unless he wishes to spend an eternity in hell. Can God simply overlook the sin? No. If God did that, He would sacrifice His justice.
Therefore, if man is to be made just, God’s justice must be satisfied. Someone must be able to pay te penalty for man’s sin. It must be a member of the offending party, the human race, but it must be one who has never fallen into the inescapable imperfection of sin. Given these requirements, no man could qualify. However, God Himself could. For this reason, God the Son came into the world and took on humanity. As the author of Hebrews says, “He had to be made like His brethren . . .” (Heb. 2:17a, emphasis added).
—R. C. Sproul, The Truth of the Cross (Reformation Trust, 2007), 90–91.