Summa summae S. Thomae sive compendium theologiae

Atheus, ἀ θεός in Greek, in Latin signifies the same as not God, or without God: whence atheism is ignorance or negation of divinity. — Hence there is a twofold species of atheism, namely negative atheism, which is nescience of God, or the lack of a notion of the existence of God, and positive atheism, which is the positive negation of the existence of God: the former arises from ignorance, the latter from error.

A twofold cognition of God can be had: one explicit and formal, by which God is cognized under the concept of Divinity, namely, as He is the supreme cause of all things, necessary being, and other things of this sort; the other implicit and virtual, which is not terminated precisely at God as He is God, or under the concept of Divinity, but at the morally good, or at another object in whose cognition God, the cause of that good or object, is necessarily involved, and consequently is implicitly cognized. — Explicit and formal cognition is again divided into distinct and confused, namely, according as His nature or quiddity, that is, the constitutive factor of the divine being, is clearly conceived; or according as it is indeed cognized that God is the supreme and universal cause, but by a generic notion which does not disclose what God is in Himself, or that from which the divine essence is constituted.

There is likewise a twofold ignorance concerning the existence of God, from which negative atheism is constituted: one vincible, which depends on the will, insofar as through either negligence or malice a man does nothing to acquire knowledge; the other invincible, which, if perhaps it exists, does not depend on the will, and cannot be avoided through means placed within man's power.

Here it is asked whether negative atheists can be given, who are ignorant of the existence of God.

This question is posed either in the order of right, namely, whether they can be given, or in the order of fact, that is, whether they are in fact found. — In the order of fact, negative atheists do not exist, for man everywhere necessarily lives in society, and there is no people among whom the idea of God, however much obscured and disfigured, is not admitted.

But can there, in the order of right, exist a negative atheist, who so invincibly is ignorant of God that he does not know Him even confusedly as the supreme and universal cause?

Three principles borrowed from the Angelic Doctor are opportunely prefigured for the response:

First: “There is present in man a natural desire of knowing the cause, when he beholds the effect... If, therefore, the intellect of the rational creature cannot attain to the first cause of things, the desire of nature will remain vain¹.” — This principle is apparent even in children, in whom there is a natural curiosity.

But that desire manifests openly that, in the apprehension of the effect, the existence of the cause is apprehended at the same time, at least confusedly, which cause is evidently correlative to the effect; otherwise that desire would not exist, if the notion of the cause were lacking, which is the terminus of that very desire.

3rd, “That which is naturally desired by man is naturally known by the same man²,” or at least can be known. For, as Cajetan most rightly notes on this passage of St. Thomas, “nature does not bestow an inclination to something to which the whole power of nature is unable to lead;” otherwise man would naturally seek something from the knowing of which he would also naturally be held back; which implies a contradiction.

These points having been set down, the question is solved.