The disbeliever rejects and insults the One True God

Messenger of Allah (S) said: Allah Almighty has said:

"The son of Adam denied Me and he had no right to do so. And he insulted Me and he had no right to do so. As for his denying Me, it is his saying: "He will not remake me as He made me at first (i.e., bring me back to life after death)" - and the initial creation [of him] is no easier for Me than remaking him. As for his insulting Me, it is his saying: "Allah has taken to Himself a son" - while I am the One, the Everlasting Refuge. I begot not nor was I begotten, and there is none comparable to Me.

[Hadith Qudsi related by al-Bukhari (also by an-Nasa'i)]