I hadn’t looked at this work for a long time, and I was struck by how much he does not blame Christianity for the decline of the Roman Empire. Here is one bit, closer to Ibn Khaldun than anything:
The rise of a city, which swelled into an empire, may deserve, as a singular prodigy, the reflection of a philosophic mind. But the decline of Rome was the natural and inevitable effect of immoderate greatness. Prosperity ripened the principle of decay; the causes of destruction multiplied with the extent of conquest; and as soon as time or accident had removed the artificial supports, the stupendous fabric yielded to the pressure of its own weight. The story of its ruin is simple and obvious; and instead of inquiring why the Roman empire was destroyed, we should rather be surprised that it had subsisted for so long. (p.435)
Gibbon also puts forward the hypothesis that basic knowledge in agriculture and the manufactures is never lost, so the overall course of history will be progressive. (pp.442-443)
He is worried about existential risk from comets, volcanoes, and earthquakes, though despairs we cannot do much about it. (p.578)
All from the Penguin abridged edition, edited by David Womersley.