„Therefore, it is necessary that infidelity should be cursed in order to serve the faith (Islam). Cursing unbelief in the heart is the lesser way. The greater way is to curse it in the heart as well as with the body. In short, cursing means to nourish enmity towards enemies of the true faith, whether that enmity is harboured in the heart when there is fear of injury from them (infidels), or it is harboured in the heart as well as served with the body when there is no fear of injury from them.
In the opinion of this recluse, there is no greater way to obtain the blessings of Allah than to curse the enemies of the faith (be impatient with them). For Allah himself harbours enmity towards the infidels and infidelity…
Once I went to visit a sick man who was close to death. When I meditated on him, I saw that his heart was layered with darknesses. I intended to remove those darknesses. But he was not yet ready for it… When I meditated more deeply, I discovered that those darknesses had gathered due to his friendship with the infidels. They could not be dispersed easily. He had to suffer torments of hell before he could get purged of them…“
— Ahmad Sirhindi Indian philosopher 1564 - 1624
Maktubat-i-Imam Rabbani translated into Urdu by Maulana Muhammad Sa’id Ahmad Naqshbandi, Deoband, 1988, Volume III, pp. 660-63. These passages are from a long letter in which Ahmad Sirhindi answered a large number of questions from his disciples.
From his letters