Divine aseity is the doctrine that God is self-existent and independent, deriving his being from no source but himself. Divine simplicity, on the other hand, teaches that God is not composed of parts; everything that is in God is God, unified in his singular essence. The link between aseity and simplicity lies in the fact that true self-existence, as aseity requires, necessitates an undivided, simple nature. Let’s explore this connection in light of biblical teaching and theological insights.

1. God’s Independence from Composition

God’s aseity is expressed in passages like John 5:26 where God the Father has life in himself and eternally gives “life in himself” to God the son. Similarly in Exodus 3:14, God reveals his name to Moses as “I AM WHO I AM.” This declaration emphasizes God’s self-existence; he is the One who simply is, uncaused and dependent on nothing outside himself (Jn 8:52). Similarly, Paul declares in Romans 11:36 that “from him and through him and to him are all things,” underscoring that God is the ultimate source of all being.

The doctrine of simplicity logically follows from this aseity. If God were composed of parts, then he would rely on something else (the union of those parts) for his existence, undermining his self-sufficiency. John Calvin affirms this, stating that God is “simple and uniform” and “free from all composition,” for “were he made up of parts, he would at once be liable to dissolution.” In other words, any internal division would imply that God is contingent upon something beyond himself for his being—a notion that contradicts his self-existent nature (Institutes, 1.13.2).

2. God as Pure Act

God’s aseity means he is fully actualized in himself, without potential or dependency. He is “the Father of lights, with whom there is no variation or shadow due to change” (Jas 1:17). If God were composed, he would have aspects or potentialities that could change, develop, or interact, suggesting unrealized or dependent potential. Thomas Aquinas builds on this in Summa Theologica, stating that God is “pure act” (actus purus) without any mixture of potentiality, because any potentiality would suggest a need for fulfillment from outside himself, violating divine aseity (ST I, q. 3, a. 7). Thus, divine simplicity affirms that God’s essence is fully actual, without parts or divisions that imply unrealized potential.

3. Unity of God’s Attributes

Aseity implies that God’s attributes are not things added to his essence but are one with his essence. In Psalm 145:3, “Great is the Lord, and greatly to be praised, and his greatness is unsearchable.” This unsearchable greatness means that God’s attributes (like wisdom, love, and power) are not external additions but are intrinsic to his being. Theologians such as Herman Bavinck explain this in terms of divine simplicity: God’s attributes are not “different in reality from his essence”; rather, “all that is in God is God.” His attributes do not constitute parts or qualities but are expressions of his unified, simple being (Reformed Dogmatics, 2:175). Therefore, there cannot be levels or degrees or contrasts between divine attributes.

4. Aseity and Divine Oneness

Aseity implies that God must be utterly one, without internal division or parts. Deuteronomy 6:4 declares, “Hear, O Israel: The Lord our God, the Lord is one.” This oneness goes beyond numerical singularity; it means that God is not divided into parts or components. Augustine explains, “God is truly and absolutely simple, without composition or division,” because any division would require external causes or combinations, contradicting his self-existence (On the Trinity, 6.6). Thus, divine simplicity reinforces God’s oneness, showing that God is not only one in number but also one in undivided essence.

In Summary

Divine aseity implies divine simplicity because true self-existence demands absolute independence, including independence from internal parts or composition. For God to be entirely self-sufficient, he must be simple—fully and entirely his own unified essence, without any internal structure that would imply reliance or division. This is why theologians, building on the testimony of Scripture, consistently affirm that God’s simplicity is necessary for his aseity, preserving the fullness of his self-sufficiency and oneness.