Question: Though the Qur'an says that it is useless to threaten disbelievers with Hell, what is the reason for Muslim preachers to deliver speeches about the eternal torment in Hell?
Yes, the Qur'an al-karim says (what means):
(Whether you threaten disbelievers [with torment] or not, it makes no difference, for they do not believe.) [Al-Baqarah 6]
It is written in the exegeses of the Qur'an al-karim that it makes no difference in terms of disbelievers, but our Master the Prophet still had to enjoin the doing of what was right and forbid the doing of what was evil, which was his duty. This sacred verse does not mean that we should not mention the terrors of the torment of Hell. If it were so, Allahu ta'ala Himself would not repeatedly have stated that Hell is eternal and its torment is very severe.
There are a lot of Qur'anic verses saying, "The torment of Hell, where disbelievers will remain forever, is very severe." Besides, our Master the Prophet was sent as a bearer of glad tidings for Believers and as a warner for disbelievers.
A Qur'anic verse says (what means):
(O Prophet! We have sent you as a witness [for Believers and disbelievers and for those who do good deeds and those who do not], a bearer of glad tidings [of Paradise for Believers], and a warner [for disbelievers against Hell].) [Al-Ahzab 45]
The basis of our religion is to learn and teach faith, the fards, and the harams. Allahu ta'ala has sent prophets for this purpose. When these tenets are not taught to youngsters, Islam will be demolished and annihilated. Allahu ta'ala commands us to enjoin what is right (amr-i ma'ruf). That is, He says that we must communicate and teach His commands. He also commands us to forbid what is evil (nahy-i munkar) and wants us not to condone His prohibitions being done.
It is declared in the following Qur'anic verses:
(Safeguard yourselves and your families from the Hellfire.) [At-Tahrim 6]
(Let there be a community among you who invite to goodness, who enjoin the doing of what is right, and who forbid the doing of what is evil. They are the ones who attain salvation.) [Al-i 'Imran 104]
Advice proves useful for Believers. They must be informed of the severity of the torment in Hell, so that they do not yearn for a life style imitative of disbelievers'. It is declared in a Qur'anic verse:
(Advise, for advice certainly benefits Believers.) [Adh-Dhariyat 55]