While God is never contingent in essence, humans always are, and the same is true of human knowledge. Aseity is, like any discussion of divine attributes, something that can be indexed in human language, gestured to, and understood in a meaningful way, but never described exhaustively. As a result, the doctrine of divine aseity resists human conquest and necessitates a sense of humility and mystery, and this is not a mark against it but a simple acknowledgment human understanding is partial.
Here are just a few resources on the central and still discussed doctrine of divine simplicity or aseity. In short, divine aseity refers to the independence of God, that he is without contingency in his divine character. It has been said that other divine attributes like eternality, immensity, and immutability derive from divine aseity, as God is not contingent in regards to time, space, and therefore does not change. This doctrine must lie at the center of Christian theology proper, even when dealing with God’s relationship to and interaction with finite and temporal human beings. It is, after all, the primary element of the Creator-creature distinction
While God is never contingent in essence, humans always are, and the same is true of human knowledge. Aseity is, like any discussion of divine attributes, something that can be indexed in human language, gestured to, and understood in a meaningful way, but never described exhaustively. As a result, the doctrine of divine aseity resists human conquest and necessitates a sense of humility and mystery, and this is not a mark against it but a simple acknowledgment human understanding is partial. The mystery begins for us as soon as a truly noncontingent God reveals himself to a contingent world of his own making.
Here is Bavinck thinking through aseity with typical clarity.
Now this independence of God is more or less recognized by all humans. Pagans, to be sure, degrade the divine by drawing it down to the level of the creature and teach a theogony; however, behind and above their gods they often again assume the existence of a power to which everything is subject in an absolute sense. Many of them speak of nature, chance, fate, or fortune as a power superior to all else; and philosophers tend to speak of God as the Absolute. In Christian theology this attribute of God was called his independence (αὐταρκεια), aseity, all-sufficiency, greatness. In the East, a number of terms were used: “(θεος ἀναρχος) God, without beginning or cause, unbegotten,” and theologians preferably spoke of God as “(αὐτογεννητος) the self-generate, (αὐτοφυης) self-begotten, (αὐτουσιος) self-existent, (αὐτοθεος) self-divine, (αὐτοφως) self-luminous, (αὐτοσοφια) self-wise, (αὐτοαρετη) self-virtuous, (αὐταγαθος) self-excellent, and so on.” All that God is, he is of himself. By virtue of himself he is goodness, holiness, wisdom, life, light, truth, and so on. As stated earlier, the church fathers usually followed Philo in grounding their description of God in the name yhwh. That was the name that described his essence par excellence. God was the Existent One. His whole identity was wrapped up in the name: “I will be what I will be.” All God’s other perfections are derived from this name. He is supreme (summum) in everything: supreme being (esse), supreme goodness (bonum), supreme truth (verum), supreme beauty (pulchrum). He is the perfect, highest, the most excellent being, “than whom nothing better can exist or be thought.” All being is contained in him. He is a boundless ocean of being. “If you have said of God that he is good, great, blessed, wise or any other such quality, it is summed up in a single word: he is (Est). Indeed, for him to be is to be all these things. Even if you add a hundred such qualities, you have not gone outside the boundaries of his being. Having said them all, you have added nothing; having said none of them, you have subtracted nothing.” Scholasticism as a whole fell in line with this view, also treating this attribute under the name of the “infinity” or “spiritual greatness” of God, or under that of the “aseity” of God, meaning that as the “supreme substance,” God is “what he is through or by his own self.” Later Roman Catholic theologians as a rule also proceeded from this aseity or independence.
In this regard the Reformation introduced no change. Luther, too, on the basis of name yhwh, described God as the absolutely existent one and as pure being. Yet, refusing to dwell on abstract metaphysical descriptions, Luther swiftly passed from “the hidden God” (Deus absconditas) to the “God revealed in Christ” (Deus revelatus in Christo). Melanchthon in his Loci describes God as “spiritual essence.” While Lutherans usually adopted this description, they often added the qualifying words “infinite,” “subsisting of himself,” or “independent.” Among the Reformed this perfection of God comes more emphatically to the fore, though the word “aseity” was soon exchanged for that of “independence.” While aseity only expresses God’s self-sufficiency in his existence, independence has a broader sense and implies that God is independent in everything: in his existence, in his perfections, in his decrees, and in his works. Accordingly, while in the past theologians mostly used the name yhwh as their starting point, in later years God’s independence occurs most often as the first of the incommunicable attributes.