Humanity is enslaved to sin and the devil by their own free choice to turn away from the Good (i.e. God) and towards evil. God could free humanity arbitrarily, but this would deny his own justice, since it was the free choice of humanity to be enslaved. The slave-master (even if it is the devil) must receive a payment for the slave.
What would he accept in exchange for the thing which he held but something... higher and better in the way of ransom. (Gregory of Nyssa, The Great Catechism, NPNF 5, 493).So the devil sees Jesus and sees one who can perform marvellous miracles; heal the sick, feed thousands from small amounts of food, and raise the dead. Looking upon this he decides that it's time for an upgrade - all of humanity in exchange for this one man.
The Enemy, therefore, beholding in Him such power, saw also in Him an opportunity for an advance... For this reason he chooses Him as a ransom for those who were shut up in the prison of death. (ibid, 493).What the devil does not realise, however, is that he's about to walk into a divinely devised deception (and Gregory is quite prepared to call it "deception" here too).
In order to secure that the ransom in our behalf might be easily accepted by him who required it, the Deity was hidden under the veil of our nature, that so, as with ravenous fish, the hook of the Deity might be gulped down along with the bait of flesh, and thus, life being introduced into the house of death, and light shining in darkness, that which is diametrically opposed to light and life might vanish; for it is not in the nature of darkness to remain when light is present, or of death to exist when life is active. (ibid, 494).As with all atonement theories this is intrinsically related to Gregory's understanding of sin and the human condition. For example, if we speak of sin as a "sickness" then atonement will taken on therapeutic terminology (e.g "healing"). If we speak of it as "guilt" or "violation of God's laws" then atonement needs to be juridical (such as penal substitution). Gregory does use other metaphors in other locations, but in this instance he is comparing "like for like". Just prior to the citations above he describes the first sin as a "deception" whereby the "glamour of beauty" was "spread over the hook of vice like a bait". As a result, Adam "enslaved himself by indulgence to the enemy of his life" (ibid, 492). Note that humanity becomes "enslaved" by sin and the devil and so the appropriate atonement metaphor uses similar terms - hence the employment of ransom terminology.
As with all atonement theories there is much to like and much more to question. The questions can wait for another day. What I do like here, however, is Gregory's underlying and fundamental belief that the presence and power of God is always superior to anything or anyone that would purport to be His opposite (there is no "opposite" to God for Gregory, by the way). Thus the "fish hook" becomes a means to an end; to deceive the devil into taking Christ into his very presence, and "give up" humanity at the same time, in order that the presence of the divinity of Christ would overcome the adversary and free humanity to live and enjoy God. The most remarkable part is just how far Gregory takes this theory (hold on to your hats here...). Read this carefully:
He who is at once the just, and good, and wise one, used His device, in which there was deception, for the salvation of him who had perished (i.e. humanity), and thus not only conferred benefit on the lost one, but on him, too, who had wrought our ruin (i.e. the devil). (ibid, 495).This is, I'm sure for many, one step too far. But we must recall the underlying belief that the presence and power of God is always superior to anything or anyone who would purport to be His opposite; including the devil. Thus, in ingesting the fish hook, the devil himself takes in Christ and is transformed for the better. It's remarkably bold and gutsy theology. We may not like it, or agree with it, but you have to respect someone willing to take such a strong belief in the power of the presence of God to this logical conclusion.
These and the like benefits the great mystery of the Divine Incarnation bestows. For in those points in which He was mingled with humanity, passing as He did through all the accidents proper to human nature, such as birth, rearing, growing up, and advancing even to the taste of death, He accomplished all the results before mentioned, freeing both man from evil, and healing even the introducer of evil himself. (ibid, 496).