One of the push back arguments against Penal Substitution is that it is making God dependent upon something. Why should we say that God “has to punish sin.” God does not have to do anything at all, he is sovereign isn’t he? The accusation here is that Penal Substitution would compromise God’s aseity. God is compelled to act.
It’s a little bit ironic that when you look at those who have aligned themselves against penal substitution that they also tend to be suspicious of classical theism criticising concepts like aseity and impassibility, seeing them as going against a loving God. There’s a leaning towards Open Theism and the God who suffers.
But let’s leave that to one side for a moment. I was fascinated to see one person comment on twitter that because of this idea that God “needed” something, Evangelicals were essentially heretical in terms of historic Christianity.
So, I thought it would be worth going back to Athanasius again, just to be reminded of what the historical orthodox have said on The Incarnation and the Atonement. On the problem of sin and death he says:
“For it was absurd that, having spoken, God should lie, in that he had established a law that men would die by death if he were to transgress the commandment, and man did not die after he had transgressed, but God’s word was made void. For God would not have been truthful, if after he had said we would die, man had not died. And furthermore, it would have been improper that what had once been created rational and had partaken of his Word, should perish and return again to non-existence through corruption. For it would not have been worthy of the goodness of God that what had been brought into existence by him should be corrupted on account of the deceit which the devil had played on men. And it would have been especially improper that the handiwork of God in mankind should come to nought, either through their neglect, or through the deceit of demons.”
Then he goes on to say:
“But as this had to be, so again on the other hand lies opposed to it what was reasonable for God, that he should appear truthful in passing the law about death. For it would have been absurd that for our benefit and permanence God, the Father of truth, should appear a liar.” 
Athanasius has a clear sense of the necessity of the Incarnation and by implication crucifixion. There is a sense in which God needed to act in a specific way. However, note that he also goes on to say:
“…pitying our race, moved with compassion for our limitation, unable to endure that death should have the mastery, rather than that His creatures should perish and the work of His Father for us men come to nought, He took to Himself a body, a human body even as our own.”
The important words there are “moved with compassion…” The point is that it is God’s own love that causes him to act. God is not dependent or compelled by an outwardly imposed need but rather it is from within his own character.
If you’ve followed on faithroots for a while, you may remember me talking about something called “The Euphythro Dilemma.” This was a conundrum set up by Plato. How do we know God is good? Is God good because he meets certain standards of goodness, abstract moral qualities such as love, justice etc or is goodness whatever God arbitrarily decides. The former means that God isn’t truly God, he is submissive to certain laws above him (so who decided those laws) or goodness is an arbitrary and therefore meaningless concept -might is right.
You may remember that we chose to refuse the dilemma. God is neither arbitrary not subject to external, abstract laws. We worship a personal God and so the qualities of goodness, compassion, mercy, kindness, justice etc are not outside and above him. Rather, they are qualities that he possesses.
Similarly, if we say that God needed to send his son and needed to punish sin then we do not mean that he needed to because of some external law of the Universe to do with justice. Rather we see God acting consistently with his character. Death could not go unpunished because
- – God is just. It is in his nature to judge and defeat evil
- – God is true and faithful. God said that death would come and so his word came true. He is faithful to his word so he will not, indeed cannot simply undo it because that would be to break with his character.
At the same time, because God is love and his justice, truth and faithfulness are loving, it is fitting and necessary to his character that he should send the Son to deliver us.
 Athanasius, De Incarnatione, s6, 149.
 Athanasius, De Incarnatione, s6, 149 -151.
 Athanasius, On the Incarnation, 2 (8).