Not only are these 144,000 explicitly "the tribes of the children of Israel" (Revelation 7:4), which no longer exist today, but they're also male virgins. If these verses are taken literally, then the 144,000 are old testament saints taken from the 12 tribes back when they existed. They died (as virgins) long ago and are now returning to earth. They're already in Heaven with seals in their foreheads before the rapture in verses 14,15 and before God's wrath is poured out in chapters 15 and 16. Also, they're the ones who preach the gospel to the world during God's wrath when the church has already been raptured. Meanwhile, the church is in Heaven getting itself ready to rule and reign with Christ during the millennium.