al-Imaam Ibnul-Qayyim (May Allah have mercy upon him) said: "So it is said: Allaah only sent His Messengers and sent down His Books with the inkaar (rejection, disavowment, refutation and negation) upon the creation due to what they were upon from the rulings of the human beings and other than that. So with this the Messengers were sent and the Books were sent down, and the abodes divided into an abode of bliss for those who make the inkaar, and an abode of misery for those upon whom the inkaar is made [i.e. those who are refuted and rejected]. So maligning and vilification in regards to that [maligning those making inkaar upon those who oppose the Truth] is maligning and attacking the Messengers and the Books...
And whoever reflects upon the conditions of the Messengers with their nations finds that they were those who established the inkaar upon them - with the strictest of establishment - until they met Allaah the Most High, and advised those who believed in them [the Messengers] with making inkaar upon those who oppose them."
[Madaarijus-Saalikeen (3/122-123)]