“Before that kafir [Guru Arjun Deva] was executed this recluse [meaning himself] had seen in a dream that the reigning king had smashed the skull of idolatry. Indeed, he was a great idolater, and the leader of the idolaters, and the chief of unbelievers. May Allah blast him! The Holy Prophet who is the ruler of religion as well as the world, has cursed the idolaters as follows in some of his prayers – “O Allah, demean their society, create divisions in their ranks, destroy their homes, and get at them like the mighty one.”
It is required by religion [Islam] that jihad should be waged against the unbelievers, and that they should be dealt with harshly… It is obligatory on Muslims to acquaint the king of Islam with the evil customs of false religions… Maybe the king has no knowledge of these evil customs… Some Ulama of Islam should come forward, and proclaim the evils present in their (unbelievers’) ways… It will be no excuse on the Day of Judgment that they did not proclaim the tenets of the Shariat because they were not called upon (to do so)…”

Maktubat-i-Imam Rabbani translated into Urdu by Maulana Muhammad Sa’id Ahmad Naqshbandi, Deoband, 1988, Volume I, pp.435-36. This letter was written to Shaikh Farid.
From his letters