Like I said before, the fact that at least two church fathers c 200 opposed christians serving in the army shows that some did or it wouldn't have been necessary to oppose this. But they tended to be exceptions, as Origen indicated in his response to a pagan critic. It's reasonable to assume even fewer christians, relative to total christian population, served after the views of the church fathers c 200 were disseminated and gradually spread. We know that they were highly influential in the church.
Except there’s no solid proof that Christians didn’t fight. As the old rule goes, a line from an ancient historian is better than a whole book of speculations from us, and as Insaid before, if Xiphilinus believed that Christians could fight in the second century for Rome, when the empire wasn’t even remotely Christian, then why wouldn’t they fight later on, when it did become Christian?
In fact christians were persecuted with state sanction from the time of Nero to about 313 CE or so. Domitian was keen on enforcing emperor worship, so the writer of revelation called him "the beast" or some such and Rome "the great harlot." Trajan said christians were not to be actively sought out but could still be punished if formally reported. There were martyrs under Pius and Aurelius etc.
As I said, sporadic and localized, every religion had its martyrs, it doesn’t mean it was actively persecuted. The general attitude of the Roman Empire prior to Decius towards Christians is best represented by Pliny’s letters. From time to time, he’d ask Trajan “what am I supposed to do with this damn Christians?” And he’d say “If they don’t cause too much of a fuss, let ‘em go, they aren’t worth the trouble”. And they weren’t, since they weren’t even that widespread until the late third century.

But "wicked emperors" were not depicted in passion scenes, just
Roman soldiers and Pilate, an agent of the Roman State. Of course christians like Lactantius and eusebius claimed not to oppose the state just certain "wicked" individuals. They
had to say that or they could get in a lot of trouble, for being openly anti-roman...
Passion scenes, and martydoms, may not have caused christians to wish for the fall of the empire. But in light of historical baggage of various kinds besides pacifism, it's reasonable to assume many christians were just
unwilling to fight like crazy to save the empire. The old spirit was gone and in light of recurrent crises which had always affected Rome, this meant doom.
And shortly after that, St Paul went ahead preaching around the whole empire, clearly believing Romans were worth the attention, constantly pointing out to every Roman that would listen that he was a Roman citizen same as everybody else. Jews saw Rome as the enemy, not Christians, and that’s what allowed them to become what they are now.
And why should we suppose that Eusebius and Lactantius were hypocrites? There were a lot of Christians that didn’t necessarily agree with Constantine and lived to tell the tale, although disgraced and branded heretics. Neither Eusebius nor Lactantius needed to write anything at all in favour of the empire, yet they did.
This is proof enough for me of their overall good will towards the institution.
No way. As Goldsworthy pointed out, while cutting off thumbs had long been a problem it appears to have become way more common in the fourth century. Roman soldiers had long been unwilling to move away from their homes but they still did it many times, down to the mid fourth century with few problems.
Honestly, I’ve based my views on the subject on what Ian Hughes wrote in his biographies of Stilicho and Aetius, plus Peter Heater in his “the fall of Rome”. I love Goldsworthy when it comes to late Republic - early empire, but when it comes to the late empire, I’m gonna stick by those who extensively wrote on the period. Besides, it’s not like Goldsworthy doesn’t blame the great estate owners same as Heater and Hughes do.
Frankly I think this is absurd. As I pointed out before, some of the worst civil wars were fought c 350-53. By that time, or for some time since Diocletian many men had been tied to the land. Yet Julian was still able to raise a very large army to invade Sassanid territory. Chronic military weakness set in only later, and coincided with the rise of christianity to prominence in the empire.
People blame the fall on economic problems, plague etc, when the Empire had endured even worse problems of these kinds prior to the fifth century and notably in the third. As long as the Roman world had strong support from its citizens it could weather any crisis. When this support waned, it rapidly went down the drain, in the west which was most heavily beset.
Yeah, and that large army, also composed of Christians like Valentinian and his brother, got trounced, and then another large army got trounced at Adrianople, and then another at Siscia, and another at Potovo, and finally another at Frigidus. There’s so much an empire split in half can take you know. Christians aren’t to blame for these gigantic losses, except maybe in the regard that the fighting armies were led by Christian generals.