Why speculate, when the ECFs and Reformers have identified it?
Several things might fit this 'restrainer' in people's minds. And how was that evil 4th kingdom a restrainer of evil, when it was an evil beast itself. Good restrains evil. Jesus said if evil is divided against itself it can not stand. Spiritual powers of wickedness dont restrain each other.
Vision of the Four Beasts - In the first year of Belshazzar king of Babylon, Daniel had a dream and visions of his head while on his bed. Then he wrote down the dream, telling the main facts. Daniel spoke, saying, “I saw in my vision by night, and behold, the four winds of heaven were stirring...
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7 “After this I saw in the night visions, and behold, a fourth beast, dreadful and terrible, exceedingly strong. It had huge iron teeth; it was devouring, breaking in pieces, and trampling the residue with its feet. It
was different from all the beasts that
were before it, and it had ten horns. 8 I was considering the horns, and there was another horn, a little one, coming up among them, before whom three of the first horns were plucked out by the roots. And there, in this horn,
were eyes like the eyes of a man, and a mouth speaking [
c]pompous words.
Vision of the Ancient of Days
9 “I watched till thrones were [
d]put in place,
And the Ancient of Days was seated;
His garment
was white as snow,
And the hair of His head
was like pure wool.
His throne
was a fiery flame,
Its wheels a burning fire;
10 A fiery stream issued
And came forth from before Him.
A thousand thousands ministered to Him;
Ten thousand times ten thousand stood before Him.
The [
e]court was seated,
And the books were opened.
11 “I watched then because of the sound of the [
f]pompous words which the horn was speaking; I watched till the beast was slain, and its body destroyed and given to the burning flame. 12 As for the rest of the beasts, they had their dominion taken away, yet their lives were prolonged for a season and a time.