The central hypothesis of The City of God by Augustine of Hippo is that the Christianization of Rome did not contribute to its decline, but in fact bought a nation already on the precipice of collapse a bit of borrowed time. The book was written in response to contemporaneous fifth century accusations of the exist opposite: that the Christianization of Rome directly contributed to its decline and overwhelming by various Goth tribes.

I've only really been able to find contemporary sources like Ryan Reeves who explicitly comment on the veracity of this claim, but I think his perspective as a Christian apologist and theologian makes his observations a bit biased (though not necessarily wrong).

I've not really been able to find any legitimate or truly scholarly sources that have anything to say other than the acknowledgement that the spread of Christianity occurred during the decline of the Roman Empire so it raises the question: Did the Christianization of Rome directly contribute to decline of the Roman Empire or was it merely happenstance that it occurred during the same period of time?

I'm aware that many Romans viewed growing Roman inability to fend off the Goths as some kind of punishment for abandoning paganism, but my question is less about what people thought at the time, and more about how historians view these events 1,500 years later. Additionally, I don't exclusively mean Constantine's conversion to Christianity as "the Christianization" of Rome, but rather the entire process (which culminated in that event of course) from the first century death of Christ onward.