I’m not sure Rome is the best example to use linking spiritual and political decline. They did pretty well as a pagan society for about 700 years. It was after they became officially Christian and banned all other religions that things really came apart.
I know what you’re saying, but remember: plenty of nations can claim the mantle of Christianity.
Whether they practice the teaching of Christ is another matter.
Rome undoubtedly turned pagan...church officials and bodies can and do become corrupt.
Jerusalem, remember, was the city of David—the temple of Yahweh was there—and it fell because the people and its leaders turned their backs on God.