Divine simplicity is the idea that God is not composed of any metaphysical parts – He is one. This is based on the Shema – ‘hear, or Israel, the Lord our God is one.’ There are two main ways of thinking about this that have dominated: the Latin tradition and the Eastern tradition – Barth’s is different from both of these.
The Latin tradition holds that God is absolutely simple in His essence – His existence is His essence, which is His power, which is His goodness, etc. God is utterly one. While we may distinguish between God’s attributes, there is no real distinction in God Himself. God is one in His simplicity.
The Eastern tradition takes a different route. God’s essence is supremely unknowable – we know God through his energies, or workings in creation. The energy/essence distinction avoids simplicity because the energies are distinct from the essence, but don’t think of this as a kind of kantian mediator, or thing-in-the-middle; God’s energies are really God, so in knowing God’s energies (such as His grace, love, mercy), we really do know God. Nothing can be said about the essence, since it is the absolute ontological chasm between God and humanity
Barth takes a decidedly different approach, known as actualism. Instead of taking the Eastern way, which he regarded as a product of subjective apophatic mysticism (he was quite wrong about the apophatic tradition, but that’s for another time), and the Western way, which he saw as abstract metaphysics, he begins with God. God as He is. As He reveals Himself. Barth refuses any kind of speculation and begins his discussion by asserting that God is the Living god, to be known on His own terms.
For Barth, the issue isn’t so much of existence and essence (though he uses that language) so much as act and being. God’s being, who He is, is what He is in His works. God’s being is in His act. His act is revelation – His self-revealing. God is who He is in His act of revelation. So, instead of God’s existence being His essence, His being is His act, and His act is revelation. God’s being is revelation, ergo, God is revelation.
So, in a nutshell, Barth basically affirms the medieval and Latin tradition of existence being essence – he simply gets in a very different way. Whereas the Latins arrive by means of metaphysics (what Barth would call speculation) Barth arrives by means of the self-revealing of the Living God. This shouldn’t be new territory for those familiar with Barth – he was famous for his rejection of metaphysics, natural theology, and speculative metaphysics about God and the nature of God. Torrance would do a good job of mellowing out Barth’s thought and giving a place to natural theology and metaphyics.
These are more or less the two dominant positions, as well as Barth’s – questions, comments, criticisms welcome.