The clergy successfully preached the doctrines of patience and pusillanimity; the active virtues of society were discouraged; and large portion of public and private wealth was consecrated to the specious demands of charity and devotion; and the soldiers' pay was lavished on the useless multitudes of both sexes, who could only plead the merits of abstinence and chastity. Faith, zeal, curiosity, and the more earthly passions of malice and ambition kindled the flame of theological discord; the church, and even the state, were distracted by religious factions; whose conflicts were sometimes bloody, and always implacable; the attention of emperors was diverted from camps to synods; the Roman world was oppressed by a new species of tyranny; and the persecuted sects became the secrets enemies of the country.Gibbon is claiming that the creation of the large Christian system 1) moved money away from more useful pursuits, 2) created factionalism and 3) distracted Romans (even emperors) from more pressing affairs of state. We know a lot more about the Roman empire now in 2012 than Gibbon did in the 18th century and we can now say with confidence that he is wrong (or at least overstating) each of these problems. At the very least, he is wrong to lay these at the feet of Christianity in some unique way.
The amount of money that was given to the church was trivial as a whole. And was probably comparable to what had been given in various celebrations and sacrifices in the pre-Christian era. Some wealth was taken out of production as churches became more ornate, but again, this was little different than what was happening with wealthy estates. If anything, the Christian practices of distributing alms to the poor, probably offset any loss.
Factionalism was alive and well before the first Christian apostles appeared. In fact, well before the time of Christ. We know this was true at the very highest levels of the Senate and throughout the peoples. It's more likely that Christianity had an overall unifying effect than otherwise.
The third claim is a bit harder to judge. Over time, the Romans certainly did lose focus on the qualities that had given them an empire. Early Roman expansion was built on, among other things, a celebration of military qualities. Was that weakened by Christian thought? Well, it certainly didn't end Roman wars. Constantine was the emperor that made Christianity an official Roman religion. He fought and others fought after him. But it has to be noted that the 'map' didn't really expand much after him.
Overall, I'm sympathetic with the notion that philosophical ideas can build up and bring down empires. I'm just not very convinced by the case that Gibbon brings forth.