The critical ontological border that was established and patrolled by Jewish monotheism was the Created versus Uncreated divide. Christianity adopted and maintained this contrast.
The doctrine in Jewish and Christian thought that established the ontological contrast between God and creation is creation ex nihilo, that God created the world "from nothing." At the time this doctrine was formalized among the church fathers there were a variety of rival cosmologies being espoused by pagan philosophers. One idea was that the world was formed by God out of some pre-existent, formless matter. Another idea was that creation was an emanation from God, that God was like the sun and creation poured out of God like sunbeams.
Creation ex nihilo pushed back on these rival cosmologies to preserve and protect the ontological contrast between God and creation. Regarding the notion that God formed the world from primordial, formless matter, where did that matter come from? Plus, how could something exist eternally "alongside" God if God did not create it? Regarding views of emanation, there were a few different concerns. A big one concerned God's free choice to create. If creation is an emanation, light flowing from the sun, then creation is a necessary outflow of God's Being rather than a free act of grace. Creation ex nihilo, therefore, was a doctrine intended to protect the idea of grace, that creation doesn't have to exist, that life is a gift. Also, to protect the ontological contrast between God and creation there had to be a time when God was shining prior to our coming into being. Phrased more simply, God's existence doesn't necessarily imply our existence. Where God exists necessarily, we exist contingently.
So that's a brief tour about what is at stake in creation ex nihilo.
I now want to turn to make a strong claim for creation ex Deo, that creation comes "from God." Due to those early concerns about emanationist theories of creation--God is the sun and we are the sunbeams--there has been much less conversation in Christian theology about creation ex Deo. And yet, creation ex Deo is the logical outcome of creation ex nihilo. If we were created from "nothing" then our being had to come from "something," and the only "something" that existed prior to our creation was God. True, there was a time when we didn't exist, as creation ex nihilo contends. There is an absolute ontological difference between God's Being and our being. And yet, our being comes from and depends upon God. More, our existence continuously depends upon God. In this sense, the emanationist vision of our relation to God has some truth. God is like the sun and we are like the sunbeams, our being continuously flowing forth from God. Creation isn't a distant, one-off event in the past. A primordial "Big Bang." Creation is, rather, our continuous ontological dependence upon God. The way the sunbeams are connected to the sun. Theologians call this creatio continua, "continuous creation."
So, our doctrine of creation begins with two ideas, creation ex nihilo and creation ex Deo. We are created from nothing and we are created from God. The former idea polices the ontological contrast between God and creation, and the latter describes our continuous ontological dependence upon God. Creation exists differently than God, but creation also exists "in" God. To make this perfectly clear, this is a panentheistic vision. Everything exists in God's own being. Everything is spiritual. As Paul declared in Acts, in God "we live, move, and have our being." And as Colossians 1 describes it, in Christ "all things hold together." God is the fabric of the universe.
To sum this up, while there are points here where theologians might quibble, my sense is that there's broad consensus about both creation ex nihilo and ex Deo, though the ex Deo part is much less described or discussed. And some theologians don't like panentheism, but I don't see how you can describe the cosmos as existing independently "alongside" God without falling into confusion and incoherence. We either exist "in" God or nowhere at all.
