This is the preface to Athanasius's much more popular book On the Incarnation. In it, Athanasius mostly makes philosophical rather than theological arguments. Whereas On the Incarnation was aimed at people claiming to be Christian (whether trinitarian or not), this one assumes nothing on the part of the hearer, not even a rudimentary theism. As such, it suffers from the same problems that all philosophical texts do, such as making starting assumptions that not everyone agrees upon. He makes the same sorts of arguments that Augustine and Aquinas would both go on to refine, ones he mostly inherited from Plato and Aristotle. Even for those of us who already agree with his conclusions, often his arguments come across as flimsy insofar as they rely on philosophy. In fact, it's remarkable how far philosophy has strayed from the starting assumptions of Athanasius, going so far today as to be radically uncertain and suspicious of even the concept of truth or discrete "selves."
A Lutheran might opt to skip this entire text and go on to its better-known sibling, primarily because Athanasius wrote: "For although the sacred and inspired Scriptures are sufficient to declare the truth..."; to some degree, it begs the question of why even waste time on this in the first place. In other words, when people's worldviews preclude supernaturalism or even teleology, why argue with them? My most charitable guess is that this is either an intellectual exercise for Athanasius, or he might be writing this for fellow believers, rather than the "gentes" (gentiles, heathens) in the title (in order to convince us our religion isn't irrational). That being said, calling this a "mere" philosophical exercise would be unfair, as Athanasius makes some interesting statements worth remarking upon.
First, we have a pre-Augustinian notion of nearness to God as "good" and departing from Him as "evil;" closely entwined with this is the notion that evil doesn't exist per se, that it is only a negation of the good, which once again Augustine is better known for elaborating on. This is central to any discussion of God, because, as Athanasius frames it, everything is always a choice. You cannot stop choosing, you cannot pause the game: "[the soul] cannot at all cease from movement, being, as I said before, mobile by nature." Related to this notion of (perpetual?) motion is an interesting argument for the existence of the soul which I hadn't yet come across: "If then the soul were moved by the body, it would follow that the severance of its motor would involve its death. But if the soul moves the body also, it follows all the more that it moves itself. But if moved by itself, it follows that it outlives the body. For the movement of the soul is the same thing as its life, just as, of course, we call the body alive when it moves, and say that its death takes place when it ceases moving."
Athanasius has a high view of the soul, primarily because it is rational. However, some medieval theologians and philosophers eventually went too far in ascribing a total ability to know God rationally rather than needing to be assisted (or completely carried) by the Holy Spirit. Here I would have to admit that the Islamic tendency toward apophatic (negative) theology is much more attractive and much safer, keeping us from falling into the pit of works righteousness which prompted the Reformation. I do think that Athanasius is at least slightly correct, however: the soul of man, being rational, can come to certain conclusions which lead it toward the bible and faith in the Christian God.
The soul of man, being intellectual, can know God of itself, if it be true to its own nature.
Neither as God Himself is above all, is the road to Him afar off or outside ourselves, but it is in us and it is possible to find it from ourselves, in the first instance, as Moses also taught, when he said: “The word” of faith “is within thy heart.” Which very thing the Saviour declared and confirmed, when He said: “The kingdom of God is within you.” For having in ourselves faith, and the kingdom of God, we shall be able quickly to see and perceive the King of the Universe, the saving Word of the Father.
It's surprising to me the wide variety of interpretations of "The Kingdom of God is within you" passage; Tolstoy takes it as an abolition of hierarchical church order, and Athanasius sees it more as an admission that everyone, thanks to their rationality, has a spark of intuition about God. Luther and other reformers likely used it in a way closer to Tolstoy, evoking the "priesthood of all believers." Taking these few examples in hand, the phase unfortunately seems to act more as a mirror of one's biases rather than any clear, positive assertion. It's one of the few teachings of Christ which is left hazy: Is it an earthly kingdom and command for social justice, as many liberal theologians argue? Is it a literal heaven and afterlife, as traditional theologians interpret it? Is it the brotherhood of believers and an abolition of religion per se, as Zizek would argue? Only the middle option is tenable, but the others are interesting to consider in tandem. Who is to say it must only have one meaning?
Returning to the issue of coming to know God, something I think that Lutheran theology glosses over to its own detriment is exactly that: how do we know God? With so much focus on giving God alone the glory, we sometimes downplay our own role so much as to make ourselves not only non-actors but almost non-entities; we are basically taught that it's our job merely to hold up the Bible, and if people reject the bible, then it's not our problem anymore. We're so averse to apologetics that we barely even evangelize, other than talking about it in the hopes that some stray fire hits the heathen. This aversion to apologetics is understandable given the heresies that arise from over-relying on reason, but even we Lutherans confess the Athanasian Creed, which uses extremely philosophical language and syntax to make its point. I don't think it's really possible to disagree with the Athanasian Creed if you read the Bible honestly, but to grant the creed is to grant also the philosophical rigor which enabled its writing and ratification.
Returning back to more general issues, I like how Athanasius chooses to attack dualism and Manichaeism: namely, the issue is how we could ever know the "Good" God if the "Evil" God is the creator of the world. Most theistic apologetics cite the world and its profound order as a central evidence of a Good God. However, when you take that away, Athanasius makes a great point: How else can we possibly know he exists? Rather than making an ordered universe, you'd expect an Evil God to create a chaotic, incomprehensible nightmare. That's decidedly not what we have. I find quite profound the argument that Athanasius makes here (and in passing in The Incarnation) that "if this were so [no Good creator God], we should consequently see not an ordered universe, but disorder, not arrangement but anarchy, not a system, but everything out of system, not proportion but disproportion. For in the general strife and conflict either all things would be destroyed, or the prevailing principle alone would appear." The last part is quite important. If the universe were created by chance, chances are (literally, all the chances except for the one which did create this universe) it would not cohere into anything, or if it did it would collapse into one oppressive excess. Unless there was intentionality, there's no way that it accidentally ends up balancing the two possible extremes into something coherent and comprehensible.
Because of the evident order of the universe, it also follows that there is a single order-er, because competing order-ers would not be able to guarantee its continued order; it would be torn apart by competing interests. This is only one of the arguments that Athanasius makes against the Roman pantheon of his day. The other main argument he makes borrows from the Old Testament, namely making fun of their idols. Rather than stopping at insulting the idols themselves because they are mute, deaf, blind, and immobile (even our technology today has gained those aspects), he continues by reviling those who worship the idol rather than the artist who made the idol. The creator of something is always greater than the thing they created (in order to create it in the first place they had to be), therefore he asks them why they don't worship the artists instead of the idols, because at least the artists made something, and often something genuinely beautiful.
I'll conclude with what is one of the most succinct ways that anyone has explained morality: "Evil, then consists essentially in the choice of what is lower in preference to what is higher." As pointed out earlier, you can never stop moving, never cease worshiping, so your choices and actions tell everyone what you value, who you fear. Athanasius's attack on the idols might seem foolish to us because it's obvious that the Romans didn't think the god was IN a little piece of wood or even a big piece of marble; rather, their gods were at the best abstract concepts like lust and gluttony, and at worst they were myths piled on top of historical men. He asks, "Why, is it fitting to regard as a god one who has perpetrated such deeds, and who stands accused of things which not even the public laws of the Romans allow those to do who are merely men?" Rather than making the gods relatable, all it does is make them beneath us. A God by definition is a transcendent being, something which forces you outside of your animal instincts and into a higher plane. This is why I don't necessarily hate religions such as the Enlightenment focus on science or the Romantic focus on art. At least these rise above the animal and attempt something higher; even if it isn't the highest Good, it's at least on the path toward Him. All we can do is keep following the path, which must lead us to Christ. Initially, it might be tempting to enter the simplicity of Islam, that "radical monotheism." However, Mohammed's self-evident moral shortcomings leave us no other choice but the Christ, who really is the only teacher throughout history I've found who actually cared for the downtrodden. No one else even comes close.