Act, because it is perfection, is not limited except by Potency, which is capacity for perfection. Therefore, in the order in which the Act is pure, it is unlimited and unique; but in that in which it is finite and manifold, it comes into a true composition with Potency.Perfection, in Thomism, means completeness. A perfect thing lacks potentiality-- it is totally what its nature is to be. Potency is the capacity for completeness, without being actually complete. A thing that is in the natural world that is limited (each thing, that is) is a composition of act and potency. Act makes the thing what it is; potency makes it limited, imperfect, and changeable (i.e. natural).
Pure Act is unlimited perfection-- absolute completeness. There can be no distinctions or multiplicity in Pure Act.
The only Pure Act is God. God is metaphysically simple, without multiplicity (there is One God) and without distinction, which is metaphysical simplicity.
How can one square Divine simplicity with the Trinity? The Persons of God are not 'parts' or essential differences within God, but are manifestations of His Personal existence. His Creative Power, His Knowledge and His Love (Father, Son and Holy Spirit) are understood by finite creatures (us) as Persons, but they are on one Simple Nature. God is Father, Son and Holy Spirit together just as He is Truth, Beauty, Goodness and Love at the same time.
The most profound truths about God cannot be known to us by direct knowledge (in this life). This kind of knowledge was called "univocal" by Aquinas, and it cannot be applied to God by us, because God's nature cannot be known or described by finite minds as He truly is. We can only describe God's Nature "analogically" (Aquinas), using finite categories to cast a dim and inadequate light on Divine Truth.
Recognition of the actual metaphysical doctrines of the Church obviates nonsense arguments about God's existence which ask "who made God" or "God must be more complex than creation and thus himself in need of a creator" (Dawkins). Such assertions aren't arguments as such, but mere admissions of ignorance about the actual metaphysical issues.