Revelation Chapter 1
Verse 7 (NASB) 
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7. Behold, He is coming with the clouds, and every eye will see Him, even those who pierced Him; and all the tribes of the earth will mourn over Him. Even so. Amen.
BEHOLD,
This is an attention getter. John is saying, Listen up! I have something important to relay to you.
He is coming with the clouds,
This is the way Daniel saw it, Daniel 7:13-14 , and as Christ Himself said it will be, Matthew 24:27-30 . The cloud has always shielded mankind from the complete and awesome power and glory of God. We cannot look upon Him directly in our current human state. But He (God) has led His children of Israel from Egypt to the promised land using a cloud. Moses was the only one to go up into that cloud to talk with God. He saw flashes of lightning from His right hand (Deuteronomy 33:1-5) just like Christ said His return would be like lightning flashes from the east to the west.
and EVERY EYE will see Him, even those who pierced Him, and all the tribes of the earth will MOURN over Him.
His coming will be visible worldwide...and what every eye will see is given in Revelation 19:11-16 . Jude in Jude 14-16 lists some of those whose eyes will see Christ's coming. He even says that as far back as Enoch (seven generations from Adam) this was prophesied. Daniel (Daniel 7:9-12) sees the coming in a spiritual light against the beast (devil).
While it is felt that the reference to "those who pierced Him" is an indictment against the Jews and Gentiles (Pilate, Annas, Caiphas, and the Jewish leaders of the Sanhedrin) directly involved with Christ's death, the immediately following phrase "and all the tribes of the earth" includes all of us in the "piercing" process.
The physical piercing of Christ's side by the Roman soldier must have made an impact on John. For he writes about it in John 19:31-37 and it shows up here in Revelation again. Whether John was aware of Zechariah's prophecy of Christ's return (Zechariah 14:4), of the wounds inflicted by "the house of my friends" (Zechariah 13:6) or of the recognition that the house of David was responsible and will mourn over Him (Christ) for having done this (Zechariah 12:10-14), is not known. At any rate, all our sins put Christ on the cross where He received this piercing; we all will mourn over Him.
Even so. Amen.References to the Revelation of Jesus Christ are in bold italics. All other references are in bold only.