Because of the pressure environment prior to the appearance of Hazrat Mahdi (as), nobody will be able to speak the name of Allah with ease

In Sheikh Tusi’s book Al-Amali: "The whole world will be filled with devastation. NOBODY WILL BE ABLE TO SAY ALLAH. Then Almighty Allah will send an individual [Hazrat Mahdi (as)] descended from me and my family. He [Hazrat Mahdi (as)] will fill the world with justice as those before him will fill it with injustice.”

(Kitab al Ghaybat, [Bihar al Anwar, Vol. 51], Ansariyan Publications, Collected by: Muhammad Baqr al-Majlis, Iran-Qum, 2003, p. 120)

In Sheikh Tusi’s book Al-Amali: "The world will be so full of oppression and injustice that NOBODY WILL BE ABLE TO SPEAK THE NAME OF ALLAH EXCEPT IN SECRET. Then Allah will bring a virtuous community (the community of the Mahdi) who will fill the world with honesty and justice in the same way it was filled with oppression and injustice before.”

(Kitab al-Ghaybat, [Bihar al Anwar, Vol. 51], Ansariyan Publications, Collected by: Muhammed Baqr al-Majlis, Iran-Qum, 2003, p. 186)