Hello miknik5,
While it is true that probably everyone who inherits eternal life will receive a white robe, not everyone belongs to the church. There are multiple groups, the church, the great tribulation saints (GTS) who become believers after the church has been removed and during the time of God's wrath, the 144,000 who come out of the twelve tribes of Israel and the woman/Israel. God has a certain number to complete the church and once that number is reached, then Lord will descend and gather the church both dead and living, which will bring the church period to a close. During that last seven years, you will have those who become believers after the church has been removed, who are the GTS, the 144,000 and Israel. None of the last three groups are ever referred to as belonging to the church.
John is not asking about the garments, but is asked by the elder who that group is wearing white robes and where they came from. Keep in mind that John was previously asked to write letters to the seven churches. You have to ask yourself the question, "why is the elder asking John who this innumerable group is." Since he was previously asked to write letters to the church, which would the elder be asking him about this group. It automatically infers that they are not the church. Also, and as I said earlier, they are never referred to as church, but saints.
Revelation 1 thru 3 = Believers are referred to as the church only. The word "saints" is not found within those same chapters.
Revelation 4 thru 18 = Believers are called saints only and the word "Church" is never used.
God did this to show a distinction between the church and the great tribulation saints, and in doing so also reveals to the reader that, from chapter 4 onward the church is no longer on the earth.
Another solid reason supporting that the church is removed before the 1st seal is opened, is because from Rev.4 onwards is a detailed account of God's wrath, which is carried out via the seals, trumpets and bowl judgments. This is significant in that, believers are not appointed to suffer God's wrath and that because Jesus already suffered it on our behalf, rescuing us from it - 1 Thes.1:10, 5:9. Which means that the church cannot be here for the seals, trumpets and bowl judgments. It would be like counting the wrath that was poured out on Christ as being of no value.