The 'trump of God' is God's own voice: Rev 1:10,11 KJV: "I was in the Spirit on the Lord's day, and heard behind me a great voice, as of a trumpet, Saying... Rev 4:1 "After this I looked, and behold, a door was opened in heaven: and the first voice I heard was as it were of a trumpet talking with me which said, Come up hither and I will shew thee things which must be hereafter". The church age of grace began on Pentecost and ends with the trump of God (I Thess 4:16-17, Rev 19:1-9). "the day of the Lord so cometh as a thief in the night" (I Thess 5:2, Joel 2:30-31, Rev 6:12-17), "For then shall be great tribulation" (Matt 24:15-21, Dan 12:7,11, Rev 11:2-3, 12:12-14, 13:5). A more accurate Bible interpretation of Daniel 9:24-27, will help to explain why only a 3 1/2 year "great tribulation". Studylight.org Daniel Chapter 9 https://www.studylight.org/commentaries/gsb/daniel-9.html . Daniel 9:24 "Seventy weeks (1 week=7 years) are determined upon thy people (Israel) and the holy city (Jerusalem)". Dan 9:25 "Know therefore and understand, [that] from the going forth of the commandment to restore and to build Jerusalem unto Messiah the Prince (Jesus Christ) [shall be] seven weeks and threescore and two weeks" = 69 Weeks. The 70th week began at the "anointing of the most holy", the baptism of Jesus Christ (John 1:29-34). Dan 9:26 "And after threescore and two weeks" (now in the 70th week) "shall Messiah be cut off (crucified)"... "(z) and the people of the prince that shall come (A.D. 70 Roman army led by Titus) shall destroy the city and the sanctuary" (Luke 21:20-24). Dan 9:27 "And he (Messiah - Jesus Christ) shall confirm the covenant (Mal 3:1, Luke 1:71-73, Gal 3:14-18) with many for one week (70th week): and in the midst of the week (3 1/2 years) he (Messiah - Jesus Christ) shall cause the sacrifice and oblation to (b) cease" (Heb 10:5-14), "(c) and for the overspreading of abominations he (Messiah - Jesus Christ) shall make [it] desolate" (Matt 23:37-38, 24:2, Luke 21:20-24). On Pentecost, in the midst of the 70th week (3 1/2 years), God established a new Dispensation - the Church age of Grace, which will last until the trump of God (I Thess 4:16-17, Rev 19:1-9). Then 'the day of the Lord - great tribulation' (Joel 2:30-31, Rev 6:12-17, Matt 24:15-21) begins and will last 3 1/2 years, and complete the 70th week; Daniel 9:27 "even until the consummation, and that determined shall be poured upon the desolate" Matthew 24:15-21, Dan 12:11, Revelation Chapters 6:12-17, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19:11-21. Rev 20:1-5 ...and judgement was given unto them: and I saw the souls of them (great tribulation saints) that were beheaded for the witness of Jesus, and for the word of God, and which had not worshiped the beast, neither his image, neither had received his mark upon their foreheads, or in their hands; and they lived and reigned with Christ a thousand years. But the rest of the dead lived not again until the thousand years were finished. This is the first resurrection"(Rev 20:4-5). The 'first resurrection' is for those that belong to Christ and occurs at various times: (I) "But now is Christ risen from the dead, and become the firstfruits of them that slept... But every man in his own order: Christ the firstfruits" (I Corinthians 15:20-23); "And the graves were opened; and many bodies of the saints which slept arose. And came out of the graves after his resurrection, and went into the holy city and appeared unto many" (Matt 27:52-53). (II) "afterward they that are Christ's at his coming" (I Cor 15:23, I Thess 4:16-17, Rev 19:1-9). (III) "Then cometh the end... when he shall have put down all rule and all authority and power. For he must reign... the last enemy that shall be destroyed is death" (I Cor 15:24-26, Rev 19:11-21, Rev 20:1-5). Rev 20:6 "Blessed and holy is he that hath part in the first resurrection: on such the second death (The Great White Throne Judgement - Rev 20:11-15) hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years".