Answer
The distinction between the economic and the immanent or ontological Trinity is the distinction that corresponds to the early church's distinction between theology proper and economy – between God in himself and what we can say about God in himself apart from creation – and God engaged with his creation – God administering his purposes in the world that he has made. This distinction has seemed to some modern theologians to have some potential pitfalls, particularly the possibility of creating a hidden God, some kind of gap between the economic Trinity that we see through God's revelation in the world and the immanent or ontological Trinity, God as he actually is in himself. What if there's a gap in there? God might seem to be benevolent toward us in Jesus Christ in the economic Trinity, but actually not be benevolently disposed to us in the immanent or ontological Trinity. So, is there a hidden God created by this apparent gap? The result is Rahner's rule, named after the famous German Catholic theologian Karl Rahner, who said the economic Trinity is the immanent Trinity and vice versa. There's no gap. If it really is divine revelation, then it is revelation of the true God. So if it's the economic Trinity that we're talking about, if it's the revelation of the Trinity, then that just does correspond to the ontological Trinity. God reveals in his redeeming work in the world in Jesus Christ who God is in himself. We don't want to have a gap that creates a hidden God. Yet, Rahner's rule needs at least one caveat, and that is that God loves us in freedom. If we look at what the Bible teaches about creation, God creates out of nothing by speaking. Creation is not a necessary emanation from God's being, creation is a free, loving, divine decision that God speaks into execution. And that establishes God's freedom from the world God has made. So, yes, if it's revelation, it's the ontological Trinity that's being truly revealed. But God didn't have to reveal himself. God lovingly chose to reveal himself through creation and redemption to the world he has made.