REVELATION 19 IS THE CULMINATION OF EVENTS FROM REV. 4 THROUGH 18. It begins with: “after these things”. That is after the Great Tribulation when the wrath of the Lamb is poured out upon His enemies.
In the 1st verse we see a great multitude in heaven shouting “Alleluia” which is a transliteration of the Hebrew word “Hallelujah” or “Praise the Lord”! This is the first time the word is used in the New Testament. You wouldn’t expect a Hebrew word to be used in the Greek New Testament, remember however, that God has been dealing with the Hebrews (Israel) during the Great Tribulation, and so after His judgments the Jews hear a shout of “Hallelujah”! Followed by “be glad & rejoice and give Him glory, for the marriage of the Lamb has come, and His Bride has made herself ready”.