Reluctance to Serve in the late Roman Army

[Epictetus]

Here is perhaps one of the most famous opinions:

The Christians were not less averse to the business than to the pleasures of this world. The defence of our persons and property they knew not how to reconcile with the patient doctrine which enjoined an unlimited forgiveness of past injuries and commanded them to invite the repetition of fresh insults… [N]or could their humane ignorance be convinced that it was lawful on any occasion to shed the blood of our fellow-creatures, either by the sword of justice or by that of war; even though their criminal or hostile attempts should threaten the peace and safety of the whole community… [T]hey refused to take any active part in… the military defence of the empire. [I]t was impossible that the Christians without renouncing a more sacred duty, could assume the character of soldiers…

Edward Gibbon, The Decline and Fall of the Roman Empire, XV.

However, at some point in time Christians were actually encouraged to fight for the Empire. This thread mentions it.