Born under the law

Does vicarious atonement require an Incarnate Deity? Apostate Dale Tuggy denies that, although he fails to explain how the death of a merely human messiah can have any vicarious significance. What makes his death different than the death of any Christian martyr? Evidently, unitarians resort to theological voluntarism. The vicarious atonement is just an arbitrary divine fiat?

Let's compare that to Christian theology. God bears a different relationship to his own law than creatures do. The Ten Commandments are inapplicable to God–not because God is amoral, beyond good and evil, but because God isn't that kind of being. Likewise, there's a fundamental asymmetry. For instance, we have a duty to worship God whereas God has no reciprocal duty to worship us!

If we keep God's law for humanity, we're just doing what we're suppose to do. And if we fall short, there's no extra credit assignment we can do to make up for our deficit. 

If, by contrast, God chooses to be born under the law, to submit himself to the penalties of his own law, that's sheerly meritorious. God had no obligation to do that in part or in whole.