You are correct in saying that by the time the events of Revelation 18:4 occur, the church will have already been raptured.  It seems certain that the Rapture takes place in Revelation 4:1-2 which says, “After this I looked, and, behold, a door was opened in heaven: and the first voice which I heard was as it were of a trumpet talking with me; which said, Come up hither, and I will shew thee things which must be hereafter. And immediately I was in the spirit: and, behold, a throne was set in heaven, and one sat on the throne.”  John the apostle, in this instance, is representative of the church and the words “Come up hither” along with the trumpet sound is indicative of the rapture of the church.  We read in 1 Thessalonians 4:16-17, “For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first: Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.” 

In chapter 17, we read about a “great whore.” (We first read of her in Revelation 14:8).  She is identified as, “And upon her forehead was a name written, MYSTERY, BABYLON THE GREAT, THE MOTHER OF HARLOTS AND ABOMINATIONS OF THE EARTH.” (Verse 5).  This woman is the exact antitype of the ancient Babylon in her idolatry and cruelty, but that ancient city called Babylon is only a picture of her in miniature.  This woman is BABYLON THE GREAT!  The inscription on her forehead is meant to show that she is not ashamed of her teachings, but publicly professes and glories in them before all the nations. 

When the Church is caught up (raptured), the papal system, or as we know it, the Roman Catholic “church” will see a great revival.  I encourage you to read chapters 17-18 to read about Babylon the Great to learn about her power, her influence, and her fall.  But to get on to your question, let’s now turn to Revelation 18:4, “And I heard another voice from heaven, saying, Come out of her, my people, that ye be not partakers of her sins, and that ye receive not of her plagues.”  This is, of course, referencing Babylon the Great and the Lord is calling His people to separate from her and all her influences.  This is actually a quote from when the Lord had called His people to separate from Babylon in Isaiah 48:20; Jeremiah 50:8; 51:6; and 51:45. Even in the Roman Catholic Church the Lord has His people: but they are in great danger; their only safety from ruin and terrible influence is in coming out of her at once. In every apostate or world-conforming church there are some of God’s people, who, if they would be safe, must come out.  Here, the harlot is not Rome alone (though she is pre-eminently so), but every church that has not Christ’s mind and spirit…every apostate church.

Since in Revelation 18, the church has already been raptured out, who are these people the Lord refers to as “my people”?  Although all believers will be taken out at the Rapture, the Lord raises up people to go forth with the Gospel during this seven-year Tribulation period.  Let’s notice that.

Revelation 7:2-5 says, “And I saw another angel ascending from the east, having the seal of the living God: and he cried with a loud voice to the four angels, to whom it was given to hurt the earth and the sea, Saying, Hurt not the earth, neither the sea, nor the trees, till we have sealed the servants of our God in their foreheads. And I heard the number of them which were sealed: and there were sealed an hundred and forty and four thousand of all the tribes of the children of Israel. Of the tribe of Juda were sealed twelve thousand. Of the tribe of Reuben were sealed twelve thousand. Of the tribe of Gad were sealed twelve thousand.”  These 144,000 Jewish believers will go out sharing the Good News during this period. 

Matthew 24-25 speaks of the events occurring during this Tribulation Period.  We read in Matthew 24:14, “And this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come.”  The Lord will have His people during the Tribulation period…Jews and Gentiles alike will turn to Him during this time of God’s wrath upon the earth.  (CC)  (562.2)