Let us take a short view of the miserable circumstances our first parents were now in. They were legally and spiritually dead, children of wrath and heirs of hell. They had eaten the fruit, of which God had commanded them that they should not eat. And when arraigned before God, notwithstanding their crime was so complicated, they could not be brought to confess it. What reason can be given, why sentence of death should not be pronounced against the prisoners at the bar? All must own they are worthy to die. Nay, how can God, consistently with his justice, possibly forgive them? He had threatened, that the day wherein they eat of the forbidden fruit, they should ‘surely die’ and, if he did not execute this threatening, the devil might then slander the Almighty indeed.
And yet mercy cries, spare these sinners, spare the work of thine own hands. Behold, then, wisdom contrives a scheme how God may be just and yet be merciful; be faithful to his threatening, punish the offence and at the same time spare the offender. An amazing scene of divine love here opens to our view, which had been from all eternity hid in the heart of God! Notwithstanding Adam and Eve were thus unhumbled and did not so much as put up one single petition for pardon, God immediately passes sentence upon the serpent and reveals to them a Saviour.
15 I will put enmity between you and the woman,
and between your offspring and her offspring;
he shall bruise your head,
and you shall bruise his heel.”
~George Whitefield~
The Sermons of George Whitefield (Wheaton, IL; Crossway Books; 2012) Sermon #1: The Seed of the Woman and the Seed of the Serpent.
The Old Guys 
Amen! We still cannot do better than some form of “Federal Headship and Theology”! But note too, I like some FV or ‘Federal Vision’ ideas, though I am not always uncritical!