This is the key verse to understanding the entire book. The major themes of the book are the second coming of Jesus, with clouds, and God's judgement on the wicked. Cross ref. Acts 1:9-11. Jesus ascends into Heaven, in a cloud, while his disciples watch. Then, two men tell them that he will return in like manner. Cross ref. Matthew 24. In verse 21, Jesus refers to the Great Tribulation. In verse 29, he refers to events immediately after the Great Tribulation, "shall the sun be darkened, and the moon shall not give her light..." (Isiah and Joel link these events to "the Day of the Lord.") Then, in verse 30, "they shall see the Son of man coming in the clouds..." [See also Mark 13:24-27 and Luke 21:25-28]