Reddit reviews The Christological Controversy (Sources of Early Christian Thought)

I started with "Christological Controversies". It's primary source documents (and the introduction provides really useful context) from the first through fourth centuries, all of which where "milestones" in the development of the Christian understanding of the divinity of Christ.

Justin Martyr's First Apology is really interesting because he's so early in the Church that he will not come right out and say "Jesus Christ is God", for example. It's not included in that book I sent you, because it wasn't a Christological work per se, but it's a good example of how pre-niceaen Catholics were talking.

The Christological Controversy (Sources of Early Christian Thought) (an accessible anthology of primary texts)

From Nicaea to Chalcedon: A Guide to the Literature and Its Background by Frances M Young

The Making of the Creeds by Frances M Young

Classical Christian Doctrine by Ronald Heine

For Us and for Our Salvation: The Doctrine of Christ in the Early Church by Stephen J. Nichols

We do, however, have to be critical of the type of witness Scripture gives us. The high Christological formulas we find in the gospels are the result of some development over the first decades after Christ's death. I say this to simply point out that statements like Thomas' shouldn't be misunderstood as implying the historical 12 disciples saw Jesus as 'homoousios' with the Father. The development of Christological and Trinitarian doctrines is a fascinating question that needs to take the historically mediated nature of revelation into account. The book I cited above is a great intro. Also, this book and this book are wonderful texts for further examination.

The Christological Controversy, an anthology of both orthodox and heretical primary sources. (There is another volume on The Trinitarian Controversy.)