In the third and fourth centuries, Lactantius and Athanasius appeared as the leading Christian thinkers about the Devil. They continued the struggle to justify belief in a personal, fallen angel Devil against the obvious holes in the argument. In doing so they succeeded in accreting yet more to the Devil idea, at times backtracking to or contradicting the arguments of previous "fathers", as well as adding their own variations on the theme.
Lactantius especially developed the idea of dualism towards its logical conclusions. Dualism was the error picked up by the Jews in captivity which influenced the first significant corruption of the Biblical concept of the Devil and Satan. They had been influenced by the old Persian idea that there is a god of evil who somehow mirrors and stands in independent opposition to the God of love. This idea remained embedded in Judaism and eventually crept into early Christianity (1). Lactantius really became obsessed with the idea, and concluded that Christ and Lucifer were originally both Angels, sharing the same nature, but Lucifer fell "for he was jealous of his elder brother [Jesus]" (Divine Institutes 3.5) . This idea meshed in with the growing departure from the Biblical position that Jesus was the begotten Son of God and as such had no personal existence in Heaven before His birth. The whole of Hebrews 1 and 2 are devoted to emphasizing the superiority of Christ over the Angels, and how He had to be human in order to save us; and that He was a human and not an Angel precisely because He came to save humans and not Angels. But that was overlooked due to the pressing need to explain how Christ and Lucifer were somehow parallel with each other. And of course Lactantius created another problem for Christianity by claiming that Christ was of the same nature with Lucifer- for if that nature was capable of sinning and falling, then what guarantee is there that one day Christ may not likewise fall, and the whole basis of our salvation come crashing down? The Persians believed that the good god would always win out over the evil god; but that was their assumption. If there are indeed these two gods, why assume one is bound to win? Not only does the Bible insist this theology is untrue (e.g. Is. 45:5-7); but if there are indeed two gods, why make the a priori assumption that the good god has to win out? What concrete evidence is there for that, beyond blind hope?
Struggling with the problem of explaining how Christ's death "destroyed" the Devil, and yet he appears alive and active, Lactantius taught that the fallen Devil had indeed been badly smitten by Christ's death, but he and his angels were gathering their forces for another assault. That runs directly against the finality with which New Testament Christianity speaks of the victory of Christ and the 'destruction' of the Devil in Heb. 2:14. The Greek katargeo translated "destroy" there means strictly 'to render useless', and is elsewhere translated in the New Testament as "make void", "abolish", "do away", "make of no effect" etc. Thus Christ will "destroy" the man of sin at His return (2 Thess. 2:8), death itself will be "destroyed" at the second coming (1 Cor. 15:26), God will "destroy" the wicked at that day (1 Cor. 6:13). Lactantius argued that the 'destruction' of the Devil at Christ's death was a temporary wound, and that he would be finally destroyed at Christ's second coming. And yet the Biblical evidence is clear that "destroy" means to render powerless. Yet Lactantius wanted to understand that when Christ 'destroyed' the Devil on the cross, that was a temporary binding; whereas at His return, the Devil would be permanently 'destroyed'. And yet the Bible uses the same Greek word to describe both destructions! The destruction of the Devil is explained by Paul, using that same Greek word katargeo, in Rom. 6:6 when he speaks of how that in the crucifixion of Jesus, and in our sharing in this by the 'death' of baptism, "the body of sin is destroyed". Yet Lactantius was following a tradition which refused to budge from the idea that the Devil exists as a personal being; and so he was forced to ignore this.
Athanasius is best known for what became known as the Athanasian Creed, a statement of the trinity. I've elsewhere argued for the deconstruction of this idea, along similar lines as I am deconstructing the personal Devil myth (2). Athanasius followed Lactantius' ideas of Jesus being in Heaven with Lucifer at the creation as part of the huge dualism which they felt existed in the cosmos- and so this meshed together with his push towards the [unBiblical] idea of a personally pre-existent Jesus who somehow became God. As with so many who've gone down blind alleys theologically, Athanasius pushed logic to an inappropriate extent rather than being guided by basic Biblical truths. He argued that the death of Jesus cleansed the air where the demons / fallen angels now live, and therefore physically opened up a way for [supposed] immortal souls to find a way into Heaven (3). Not only was all this unBiblical, it reflects a literalism which reduces God to a being hopelessly bound by physicality. In short, this kind of thinking arose from a basic lack of faith in God as the Almighty, who doesn't need to build bridges over problems which men have created for Him in their own minds. It should be noted that the idea of saying "Bless you!" when someone sneezes derives from Athanasius' idea that demons can become so small that they enter a person from the literal air. I consider Athanasius' misuse of Paul's reference to "the prince on the power of the air" in section 5-23. It should be noted that in the 17th century, Isaac Newton rejected the popular idea of the Devil and demons, and in his "Paradoxical questions concerning Athanasius", Newton blames Athanasius as being especially responsible for introducing this false idea into popular Christianity.
Athanasius was led by his views on Satan to de-emphasize human sinfulness. He placed the blame for Adam’s sin so fully upon Satan that he concluded that we can live entirely sinlessly- he claims Jeremiah and John the Baptist did so, even though they lived before the death of Christ (4). So one error lead to another; by de-emphasizing the weight and seriousness of human sin, he de-emphasized the meaning and crucial achievement of the cross. Perfection was not possible for those under the Old Covenant; if it had been, then there would have been no need for the priesthood of Jesus- so reasons Heb. 7:11. In his zeal to excuse human sin and blame it all on Satan, Athanasius missed this point- and it just happens that this point is the very crux of Christianity. And this de-emphasis of human sin continued in the thinking of the later ‘church fathers’. Pelagius insisted that Christians could become without sin: “A Christian is he who imitates and follows Christ in everything, who is holy, innocent, unsoiled, blameless, in whose heart there is no malice... he is a Christian who can justly say ‘I have injured no one, I have lived righteously with all’” (5). Whilst these are all Biblical ideals, this sickening self-righteousness is a far cry from the desperation of Paul in Romans 7, where perhaps the greatest of Christians admitted he constantly did the things he hated doing. It was this de-emphasis upon sin which resulted in the image of Christianity being developed as white-faced, pious, hypocritical, self-righteous, self-commending etc. And I submit this tragically deformed version of Christianity all began with a de-emphasis of human sin, and the misunderstanding of the nature of being human which goes with faulty belief about Satan.
Notes
(1) There is a wide literature on how Persian dualism influenced Judaism and thence entered Christian thought. See, e.g., Abraham Malamat, History Of The Jewish People (London: Weidenfeld, 1976) and John R. Hinnells, Persian Mythology (New York: Bedrick Books, 1985).
(2) See my The Real Christ.
(3) This and other Athanasius references from Nathan K. Ng, The Spirituality of Athanasius (Bern: Lang, 2001).
(4) Quotations in J.N.D. Kelly, Early Christian Doctrines (London: A. & C. Black, 1968) p. 348.
(5) Quotations in Kelly, ibid p. 360.