On Mount Sinai, just before the giving of the Ten Commandments, Moses "led the people out of the camp to meet with God, and they stood at the foot of the mountain." This scene from the book of Exodus (19:17) fairly bristles with details that early Christians would later associate with the coming of Jesus Christ, the resurrection and the last judgment. The Israelites were to "be ready" (Ex 19:11), and Jesus told his disciples to be ready for his coming (Mt 24:44). Moses told them to prepare themselves by abstaining from sexual relations (19:15), and Paul urged this standard on his converts because the end was so near (1 Cor 7:1, 8, 29-32). The Israelites were waiting to "meet with God," and Paul expected Christians to "meet the Lord in the air" (1 Thess 4:17). The Sinai revelation was to happen on "the third day" (19:11, 16), and Jesus rose from the dead the third day (for example, 1 Cor 15:4; Mt 16:21; Lk 13:32).
Most significant for an understanding of John's visions in Revelation is that the Israelites saw "thunder and lightning, with a thick cloud over the mountain" and heard "a very loud trumpet blast" (Ex 19:16). When the people gathered, "Mount Sinai was covered with smoke, because the LORD descended on it in fire. The smoke billowed up from it like smoke from a furnace, the whole mountain trembled violently, and the sound of the trumpet grew louder and louder" (19:18-19). The letter to the Hebrews rehearses the whole scene at Sinai, including the "mountain . . . burning with fire," the "darkness, gloom and storm," and the "trumpet blast" (Heb 12:18-19), with a promise that earth and heaven would be shaken again one last time (12:26-29).
For Paul, the trumpet in particular became part of the scenario of Jesus' return, along with the resurrection of believers in 1 Thessalonians 4:16 ("with a loud command, with the voice of the archangel and with the trumpet call of God, and the dead in Christ will rise first") and 1 Corinthians 15:51-52 ("in a flash, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, the dead will be raised imperishable, and we will be changed"). John, in the same tradition, combines the imagery of "fire from the altar" hurled to earth and "peals of thunder, rumblings, flashes of lightning and an earthquake" (8:5) with the familiar trumpet blast, now "serialized" as seven trumpets in keeping with the sevenfold character of many of his visions.