Adam Clarke on Matthew 24:30

Then shall appear the sign of the Son of man - The plain meaning of this is, that the destruction of Jerusalem will be such a remarkable instance of Divine vengeance, such a signal manifestation of Christ's power and glory, that all the Jewish tribes shall mourn, and many will, in consequence of this manifestation of God, be led to acknowledge Christ and his religion. By της γης, of the land, in the text, is evidently meant here, as in several other places, the land of Judea and its tribes, either its then inhabitants, or the Jewish people wherever found.