Atheist, the Eternalist
فَأَمَّا كُفْرُ الْجُحُودِ فَهُوَ الْجُحُودُ بِالرُّبُوبِيَّةِ وَ هُوَ قَوْلُ مَنْ يَقُولُ لَا رَبَّ وَ لَا جَنَّةَ وَ لَا نَارَ وَ هُوَ قَوْلُ صِنْفَيْنِ مِنَ الزَّنَادِقَةِ يُقَالُ لَهُمُ الدَّهْرِيَّةُ وَ هُمُ الَّذِينَ يَقُولُونَ وَ ما يُهْلِكُنا إِلَّا الدَّهْرُ وَ هُوَ دِينٌ وَضَعُوهُ لِأَنْفُسِهِمْ بِالِاسْتِحْسَانِ عَلَى غَيْرِ تَثَبُّتٍ مِنْهُمْ وَ لَا تَحْقِيقٍ لِشَيْءٍ مِمَّا يَقُولُونَ
(In a lengthy Hadith from 6th Imam-asws) So, as for the Kufr of the denial, so it is the denial of the Lordship (of Allah-azwj), and it is the speech of the one who is saying, ‘There is neither a Lord-azwj, nor a Paradise, nor a Fire, and it is the speech of two types of ‘الدَّهْرِيَّةُ’ the atheists called the ‘Dahriyya’ (Eternalists); and they are the ones who are saying, [45:24] and nothing destroys us but the passage of time’; and it is a religion placed for their own selves with the approval upon without there being an affirmation from them, nor any investigation of anything from what they are saying.
قَالَ اللَّهُ عَزَّ وَ جَلَّ إِنْ هُمْ إِلَّا يَظُنُّونَ أَنَّ ذَلِكَ كَمَا يَقُولُونَ وَ قَالَ إِنَّ الَّذِينَ كَفَرُوا سَواءٌ عَلَيْهِمْ أَ أَنْذَرْتَهُمْ أَمْ لَمْ تُنْذِرْهُمْ لا يُؤْمِنُونَ يَعْنِي بِتَوْحِيدِ اللَّهِ تَعَالَى فَهَذَا أَحَدُ وُجُوهِ الْكُفْرِ
Allah-azwj Mighty and Majestic Says: they only but conjecture [45:24], if it was like as what they are saying. And He-azwj Said And it is alike to them whether you warn them or do not warn them, they will not be believing [36:10]. Meaning, (believing) in the Oneness of Allah-azwj the Exalted. Thus, this is one of the aspects of the disbelief (Kufr)’ (an extract).[1]
NB: https://hubeali.com/atheist-mamomeen-asws-replies-to-them/
[1] Al Kafi V 2 – The Book Of Belief and Disbelief CH 166 H 1