John the revelator in chapter 7 saw the slain under the alter and did not know who they were. He cried out to the : "who are these"?.John knew the church in Heaven, for he was given the vision as part of Christ's raptured church, but these he did not know slain and now under the alter."When he opened the fifth seal, I saw under the altar the souls of those who had been slain because of the word of God and the testimony they had maintained".Verses 13-14
One of the elders comes over to John. He asks John who these
clothed in the long white robes are and where they came from?This elder seems to think John ought to know who they are.
But John says Sir Thou knowest. It is then that the elder tells
John what he is witnessing.God is giving John a vision of things that are to happen thereafter. The day would come
when a numberless multitude would stand before God having just
entered heaven. They would be the multitude THAT CAME OUT OF
the great tribulation. John sees this multitude in heaven
which meAns that the antichrist has killed them, attempting to
wipe every believer off the face of the earth.