Atheism, in modern terminology, means denying the existence of Creator altogether, denying that He exists and not acknowledging Him, may He be glorified and exalted. The universe and everything in it, according to their claims, came about purely by chance. This is a strange view which is contrary to sound human nature, reason and logic, and is contrary to simple logic and indisputable facts.
As for shirk is concerned(polytheism) it implies that a person believes in creator and affirms of Him but it also includes belief in a partner to Allah. They associate some other person as a diety equivalent to Allah, who creates or grants provision or brings benefit or wards off harm. This is shirk al-ruboobiyyah (ascribing partners to Allah in His Lordship). Or it means belief in a partner to whom some kind of worship is devoted as an act of love and veneration, as it is devoted to that person or thing as it should to devoted to Allah, may He be glorified and exalted. This is shirk al-‘ibaadah (associating others with Allah in worship). By studying these two deviations, we may see that each of them involves sin and evil which tells us that they are bad and we see how Allah described them as being like dumb animals.
Allah says (interpretation of the meaning):
“Have you (O Muhammad صلى الله عليه وسلم) seen him who has taken as his ilaah (god) his own vain desire? Would you then be a Wakeel (a disposer of his affairs or a watcher) over him?
44. Or do you think that most of them hear or understand? They are only like cattle nay, they are even farther astray from the Path (i.e. even worse than cattle)”
“And surely, We have created many of the jinn and mankind for Hell. They have hearts wherewith they understand not, and they have eyes wherewith they see not, and they have ears wherewith they hear not (the truth). They are like cattle, nay even more astray; those! They are the heedless ones”
Nevertheless, the atheist who denies the existence of Allah and rejects His Messengers and disbelieves in the Last Day, is in a greater state of kufr and his beliefs are more reprehensible than the one who believes in Allah and the Hereafter, but he associates something of His creation with Him. The former is stubborn and arrogant to an extent that can not be imagined or accepted by sound human nature. Such a person would transgress every sacred limit and fall into every sin; his worldview would be distorted to an inconceivable level. Yet many scholars who discussed the issue of atheism doubted that this has deep roots in the hearts of the atheists, and they affirmed that the atheist is only professing atheism outwardly; deep down he believes in one God.
Shaykh al-Islam Ibn Taymiyah said a great deal which indicates that this group of atheists who deny and reject the existence of God are in a worse state of kufr than the mushrikeen who associate partners with Him. We will quote a little of what we have come across:
He (may Allah have mercy on him) said: Kufr (disbelief) means not believing in Allah and His Messengers, whether it involves rejecting or it consists of doubt and uncertainty about the issue or ignoring the issue altogether, out of envy or arrogance or following whims and desires that divert a person from following the Message. However, the kaafir who rejects and disbelieves is in a state of greater kufr, although the one who rejects and denies out of envy, even though he believes that the Messengers brought the message of truth, is also in a state of kufr.
He also said:
The one who denies the Hereafter but believes that this universe is created is described by Allah as a kaafir. The one who denies it and says that this universe existed from eternity is a worse kaafir in the sight of Allah, may He be exalted.
He said (may Allah have mercy on him) refuting those who deny the Divine attributes:
(Denying the Divine attributes) implies complete denial which reaches the point that says: There is nothing that must exist and cannot have not existed. If he believes that and says: I do not affirm either existence or non-existence, then the answer to that is: Suppose you state that verbally and in your heart you do not believe either of the two; rather you turn away from knowing Allah and worshipping and remembering Him, so you never remember Him, worship Him, call upon Him, put your hope in Him or fear Him; (in that case) your denial of Him is worse than that of Iblees who (at least) acknowledged Him. End quote.
And he (may Allah have mercy on him) said:
The arrogant one is the one who does not acknowledge Allah outwardly, like Pharaoh. He is in a worse state of kufr than them (meaning the mushrik Arabs). Iblees, who enjoins all of that and loves it and is too arrogant to worship his Lord and obey him, is in a worse state of kufr than them (the mushrikeen), even though he was aware of the existence and might of Allah, just as Pharaoh was also aware of the existence of Allah.
He also said:
The view of the philosophers -- those who say that the universe is eternal and that it is dependent on that which inevitably must exist -- came from the minds and hearts of those who worshipped heavenly bodies and made images of them on Earth, such as Aristotle and his followers. This view is worse kufr and is more misguided than that of the Arab mushrikeen who believed that Allah created the heavens and the earth and everything between them in six days by His will and power, but they attribute falsely without knowledge sons and daughters to Him (cf. al-An’am 6:100) and joined others in worship with Allaah, for which He had sent no authority (cf. Aal ‘Imraan 3:151). Similarly, the permissive people, who do not believe in any command or prohibition at all and refer to the Divine will and decree as an excuse for their evil deeds, are worse off than the Jews, Christians and Arab mushrikeen, because even though the latter are kaafirs, they still believe in some kind of command and prohibition, and the promise and warning (i.e., the Hereafter), but they had partners with Allaah (false gods) who instituted for them a religion which Allaah had not ordained (cf. al-Shoora 42:21), unlike the permissive people who ignore all laws altogether. They are only pleased with whatever suits their whims and desires, and they get angry on the basis of their whims and desires; they do not get pleased for the sake of Allah, or angry for the sake of Allah, or love for the sake of Allah, or hate for the sake of Allah; they do not enjoin that which Allah has enjoined and they do not forbid that which Allah has forbidden, unless that suits their whims and desires, in which case they do it for that purpose and not as an act of obedience to their Lord. Hence they do not denounce what takes place of kufr, evil doing and sin unless it goes against their whims and desires, in which case they will denounce it, prompted by their devilish nature and not prompted by sharee’ah and love of Allah. Hence the devils plunge them deeper into error, and they never stop short (cf. al-A’raaf 7:202), and the devils may show themselves to them and address them and help them with some of their whims and desires, as the devils used to do with the mushrikeen who worshipped idols.
Shaykh Ibn Baaz (may Allah be pleased with him) said: It is shirk to worship something other than Allah completely; that may be called shirk or kufr. Whoever turns away from Allah altogether and directs his worship to something other than Allah, such as trees, rocks, idols, the jinn or some of the dead, those whom they call awliya’ (“saints”), worshipping them or praying to them or fasting for them, and forgetting Allah altogether -- and this is the worst kind of kufr and shirk. We ask Allah to keep us safe and sound.
The same applies to denying the existence of Allah and saying that there is no God and life is material, like the Communists and atheists who deny the existence of Allah. These are the worst disbelievers among mankind, the most astray, the most involved in shirk and the most misguided. We ask Allah to keep us safe and sound. End quote.
He also said (may Allah have mercy on him):
Meat slaughtered by Communists is haraam and is like the meat of the Magians and idol worshippers; in fact their meat is even more haraam, because their degree of kufr is greater due to their atheism and denial of the Creator (may He be glorified and exalted) and His Messenger, and other kinds of kufr.
And Allah knows best.