Behold, he cometh with clouds - This relates to his coming to execute judgment on the enemies of his religion; perhaps to his coming to destroy Jerusalem, as he was to be particularly manifested to them that pierced him, which must mean the incredulous and rebellious Jews.
And all kindreds of the earth - Πασαι αἱ φυλαι της γης· All the tribes of the land. By this the Jewish people are most evidently intended, and therefore the whole verse may be understood as predicting the destruction of the Jews; and is a presumptive proof that the Apocalypse was written before the final overthrow of the Jewish state.
Even so, Amen - Ναι, αμην· Yea, Amen. It is true, so be it. Our Lord will come and execute judgment on the Jews and Gentiles. This the Jews and Romans particularly felt.