If Christianity did not cause Rome to fall, then why do the accounts of Roman pagan aristocrats directly blame the decline of Rome on its adoption of Christianity and the subsequent loss of its Roman-ness? The people that lived during the decline of the Roman empire would surely know better than any of us on why Rome had declined after all. Roman social circles and institutions were tied to the culture of the Roman people, which includes its intellectual and spiritual life, as well as all its philosophy, pagan rites/rituals, warrior ethos, and so on. The abandonment of this culture left Rome vulnerable to outsider invaders.
Christians are now experiencing the same thing that the Romans experienced, and the Roman people will have their revenge from beyond the grave. The Romans were good to the Christians, and in turn the pushy and preachy Christians acted like victims when they were powerless and then turned against their ancestral religion once powerful, destroying their own temples and banning Roman beliefs entirely. We can even see today how Christian thought and culture pacifies people who are being invaded by masses of foreigners.