Theodosius I signs Edict of Thessalonica in 380 CE, making Christianity the official state religion of the Roman Empire.

In 395, after winning two destructive civil wars, Theodosius I died. He left a collapsing field army, and the Empire divided between the warring ministers of his two incapable sons. Goths and other non-Romans became a force that could challenge either part of the Empire. Further barbarian groups crossed the Rhine and other frontiers. The armed forces of the Western Empire became few and ineffective, and despite brief recoveries under able leaders, central rule was never again effectively consolidated.

Wikipedia - Fall of the Roman Empire

So my question is this - there's only 15 years between the acceptance of Christianity as the state religion and the collapse of the state. What were the effects of forced conversion in the late western Roman empire, and did it accelerate or delay the fall of the empire?

I haven't read much about the theological dimensions of the later stages of the Roman Empire. But I would contest the premise of the question. Instead I would argue that the Western Roman Empire more or less continued until the death of Theoderic the Great in 526. Yes, in name he wasn't Roman Emperor, but only because of the might of the Eastern Roman Empire, whose regents would have been irate if he called himself that. But he was considered to be a Roman Emperor by many Roman citizens of the time, not only largely accepting him, but in many cases even celebrating him for trying to restore old buildings, roads, institutions etc.

Too much Gibbon, and pretending that the East wasn't the Roman Empire.

Power had been consolidating in the East for well over 100 years, and the emperors who "fixed" the crisis of the third century, from Aurelian to Diocletian, were all Easterners, some of whom never set foot in Rome. If anything, the rise to prominence of an Eastern cult that had gained popularity with Eastern troops reflected that decline of the importance of the west rather than having much effect on it.

Ok. While I pride myself on history. Can you like ELI5 this a little bit? Not that you need to it's probably some reader's won't get it. Or find a different point of view.