St Paul’s second epistle to the Thessalonians is a locus classicus:-

And you know what withholds, that he may be revealed in his time. For the mystery of iniquity already works; only he who now holds is to hold until he be taken out of the way. And then that wicked one shall be revealed (2 Thess. 2).

Bellarmine comments:-

Here Paul speaks, not venturing to write openly about the overthrow of the Roman Empire, which nevertheless he had clearly explained when he was with them, and the meaning is: ‘Do you know what hinders the coming of antichrist? I told you, the Roman Empire hinders it, because its sins are not yet completed, and Antichrist, who will take this empire out of the way on account of its sins, will not come until they are completed. And so the one who now holds is to hold, that is, to reign, until he be taken out of the way, that is, abolished, and then that wicked one will be revealed (Tomus II, Liber III, caput V).

As Bellarmine shows in loc., this is a common view among the fathers, even though Augustine acknowledges that other interpretations are possible.  That the Roman imperial power was the historic force preventing Christ’s enemies from coalescing under one visible head was believed, strange as it may seem, even when the emperors were pagans and persecutors. It was still believed, naturally enough, when the emperors became the Church’s patrons   and protectors. It is as if, the birth of our Lord having been heralded by the decree going forth from Caesar Augustus, Caesar Augustus’s power must disappear before the anti-decree will go forth. But has not this been, as Maritain wrote in his study of St Paul, disproved by the facts?

The Eastern Roman Empire continued until Whit Tuesday, 1453. Within a lifetime the Protestant revolt had begun: the greatest undermining of the faith yet known. The Emperors of the West continued to have the Roman name until August 6th, 1806. In the lifetime that followed this, the basis of natural religion, namely belief in a personal God, ceased to be part of the general heritage of mankind. Insofar as the Roman imperial power and tradition continued, it was vested in the Austrian emperors until Hallowe’en 1918. In the lifetime that followed this date, natural law was destroyed. The Fathers, and Bellarmine, do not say that Antichrist would appear the day after the dethroning of the last emperor, but that the imperial power was preventing his arrival. Anti-Christian forces would always be at work, but independently one of another; once the imperial power was removed, they would, somehow, be freed to co-ordinate their efforts in view of a supreme attack. Finally, the last heir apparent to a reigning emperor died last year on Independence Day. It remains to be seen what will happen next.