“Ilhad” of Allah’s Names
In our times, an atheist is commonly labeled as a “mulhid” (ملحد) in Arabic; however, the Qur’an also speaks about a different category of “mulhideen”.
Allah subhanahu wa-Ta’ala says:
وَلِلَّهِ الأَسْمَاءُ الْحُسْنَى فَادْعُوهُ بِهَا وَذَرُوا الَّذِينَ يُلْحِدُونَ فِي أَسْمَائِهِ سَيُجْزَوْنَ مَا كَانُوا يَعْمَلُونَ
And to Allah belong the best names, so invoke Him by them. And leave [the company of] those who practice deviation concerning His names [yulhidun fi Asmaa’ihi]. They will be recompensed for what they have been doing. [7:180]
So here Allah is speaking about those who do “ilhad” of His names. As az-Zamakhshari and other Qu'ranic exegetes have noted, this could refer to those who do acknowledge the existence of God, yet fail to affirm for Him the positive attributes He is worthy of (e.g., Mercy) and can even go as far as affirming negative attributes for Him (e.g., injustice, etc.).
I believe this is an apt description of deists who, despite acknowledging God’s existence, fail to glorify Him and recognize Him for being a Merciful, Just, and Wise God. In my opinion, a deist is an exemplar of such a “mulhid" in the names of Allah. In fact, all false religions are deficient in their theology because they suffer from the “ilhad” of Allah’s names in some form or another, whether it is primarily in theory (e.g., Biblical Unitarianism), application (e.g., Judaism), or both (e.g., Christianity).
Further Reading on Deism: A Critique of Deism | Bassam Zawadi