What is the ruling regarding one who said: “The Quraysh are hypocrites,” then cursed the forefathers of the Quraysh and praised the descendants of Isa (Jesus), peace be upon him (the Christians of today)? And is it permissible to warn against him?

Hypocrisy (nifaq) was not known in Quraysh at the time of the Prophetic message. Accusing them of hypocrisy is baseless. Cursing their ancestor Fihr or An-Nadr is impermissible due to the possibility of him having embraced Islam. If "fatherhood" is meant to signify religious fatherhood, then cursing the religious leaders of Quraysh, such as the Prophet (peace be upon him), is an act of disbelief (kufr), and intending to curse the Companions is a grave sin. Praising Christians is baseless because they are astray, and warning against them is required.