The saints “under the altar” in Rev 6 are clearly identified as those who had been slain for the Word of God and the testimony they maintained, and yet they do not have their resurrection bodies. It sounds like Christians will be martyred during the seal judgments. Doesn’t that indicate the rapture hasn’t happened yet?
The martyrs under the altar are Christians but that doesn’t make them part of the Church. Multitudes of people from all around the world will die during the seal judgments. Many of them will have become believers after the rapture (Rev. 6:9, Rev, 7:9). Theologians sometimes refer to them as Tribulation Martyrs.