“when the trumpet is sounded, it will be a day of distress, not at all easy for the disbelievers”
Regarding the above verse, Imam Sadiq (as) said, There will be a victorious Imam from among us who will be hidden (from the eyes of the people). When Allah wills for him to reappear, He will send an inspiration to (the Qaem’s) heart and he will rise with the order of Allah. (Al Kafi volume 1 page 343)
Imam Baqir (as) explained the verse, “For when the trumpet is sounded.” He (the Imam (as)) said, “The trumpet” refers to the announcement from the heavens which will announce, “Verily your Wali (master) is someone, son of someone. He is the Qaem (the rising Imam) who has come with the truth.”
This announcement will be made by Jibraeel three hours before the rising of the Qaem (as)
“It will be a day of distress, not at all easy for the disbelievers”. “The disbelievers” refer to the Murji’ah, who are those who disbelieved in the Grace of Allah and in the Wilayat of Ali bin Abi Talib (as). (Taweel Al Ayat Al Zahirah page 708)