Zechariah 6:7

And the strong went forth and sought to go, that they might walk to and fro through the earth - The mention of their strength corresponds to the extent of the power and commission, for which they asked, to go to and fro, up and down, at their will, unhindered, through the whole earth. The Babylonian empire held Egypt only out of Asia; the Persian was conquered in its efforts against Europe, in Greece; Alexander’s was like a meteor, gleaming but breaking into the four: the Roman combined East and West and within large limits tranquilly.

And he said go, walk to and fro in the earth - He commanded, and they, which were before withheld, went, “and they walked to and fro on the earth,” ordering all things at their will, under the Providence of God, whereby He gave free access to the Gospel in all their wide empire. The Greek empire being extinguished, the Romans no longer went into any given country, but superintended and governed all human things in (it is the language of the New Testament) “all the world.” Cyril: “These same, the dappled and ashen-grey horses were commanded to traverse the earth, and they did traverse it; for they mastered all under heaven, and ruled the whole earth, God consenting and arraying those who swayed the Roman might with this brilliant glory. For, as God, He knew beforehand the greatness of their future piety.”