Aquinas offers a very compelling account of how to reconcile the transcendence of God with His immanence. This reconciliation is most compelling because Aquinas claims that God is most transcendent from, and most immanent in, creation for the very same reason, i.e. because God is Ipsum esse subsistens (Subsistent Act of Existing Itself).
According to Aquinas, all creatures are fundementally composed of essence and existence. He argues that because
one can know the essence of a created thing, i.e. what a thing is, without thereby knowing anything about whether it exists in reality, essence differs from existence. Thus, every created thing is a composition of what it is and an act of existing whereby it is a real, actual thing. (For more on this, see On Being and Essence, Chapter 4.) This act of existing, esse, is sometimes refered to as that feature about things that makes them something rather than nothing, or real as opposed to imagined. (I don't think that these ways of speaking are enough to prove the reality of the distinction, but they illustrate the point.) Since every creature is a composition of essence and esse, there must be a First Cause of this composition that is Himself uncomposed. In God there is no distinction between What He is, i.e. His Essence, and the act wherby He is. (Summa Theologiae Ia, 3, 4) Thus, because God is utterly simple, and not composed, and utterly perfect, he utterly transcends every creature.
However, since no creature exists through itself of itself, every creature is continually kept in existence through continual active causality of God. God is the cause of the being of all things precisely because He is Subsistent Being Itself (ipsum esse subsistens)
Now, since God is being itself by His own essence, created being must be his proper effect....Therefore, as long as a thing has being, so long must God be present to it, according to its mode of being. But being is innermost in each thing and most fundementally present within all things.... Hence it must be that God is in all things and innermostly. (S.T. Ia, 8, 1)
Hence, by one and the same principle, that God is "impsum esse subsistens," Aquinas is able to claim that God is at once most trancendent, and infinitely so, from creatures, and that God is most immanently active in his creation. Although God, as Aquinas conceives him philosophically, is utterly Other than his creation (as the Judeo-Christian tradition maintains) He is no Divine Watchmaker, who set the cosmos running but has no further connection with his creation. Rather, for Aquinas, God is "innermostly" present precisely because He is utterly Other.
But with all this talk of God being "ipsum esse subsistens," Aquinas believes that we should be on guard against two mistakes.
First, that God's transcendence means that He is utterly unknowable. Aquinas' claim that God is utterly transcendent is a conclusion and corollary of arguments that all things are caused by God. Nevertheless,
Aquinas believes that creatures can provide us with genuine knowledge about God.
Hence from the knowledge of sensible things the whole power of God cannot be known; nor therefore can His essence be seen. But because they are His effects and depend on their cause, we can be led from them so far as to know of God whether He exists, and on to know of Him what must necessarily belong to Him, as the first cause of all things, exceeding all things caused by Him. (S.T. Ia, 12,
In a way, the only positive knowledge contained in the claim "God is transcendent" is really a claim about creatures: "The cause of creatures is utterly unlike creatures."
This brings us to the second mistake against which we should guard ourselves. It would be a mistake to think that we know what we are talking about when we say that God is His own act of existence. For Aquinas, we know that such a claim about God is true since it follows from what we know about creatures.
But we do not know what "His own act of existence" is, i.e. we do not know His nature.
Thus, Aquinas' metaphysics offers a way to say a great deal about God and his nature, based on reasoning that ultimately begins with sensible creatures about which we have some genuine metaphysical knowledge. We can come to real philosophical and metaphysical knowlegde of God (that He exists, that He is one, unique, good, intelligent, spirit, loving). Yet for all this natural knowledge, Aquinas consistently maintains there are "truths which exceed human reason" (S.T.
Ia, 1, 1) that we could NEVER know about His nature (that He is a Trinity of Persons, that He became a man as Jesus Christ, that He offers Redemption through Christ) if He did not reveal them to us. Such truths constitute Sacred Doctrine which is superior to philosophy because it is knowledge of God "so far as he is known to Himself alone and revealed to others" (S.T. Ia, 1, 6)