The Book of Revelation (4): The Interlude of Revelation 7
After the first 6 Seals are broken at the start of the Tribulation, releasing the Birth Pains, there is an Interlude before the 7th Seal releases the 7 Trumpets. During this Interlude (described in Revelation 7) the 144,000 from 12 tribes of Israel are prepared and sealed to spearhead evangelism in the Tribulation.