Difference Between Kufr & Shirk
Q “I have a three-part question: Is there a difference between Kufr (disbelief) and Shirk (Polytheism)? What is the difference between the kaafireen (disbelievers) and the mushrikeen (polytheist)? Whoever does a nullifying action from the nawaaqidul Islaam (nullifiers of Islam) and then repents after that, is there taubah (repentance) for this individual?”

A Yes, al-Kufr (disbelief) is broader and more general than ash-shirk (polytheism) and that is because disbelief can be due to denial of the Lord; (Rabb Subhaanahu wa Ta’aalaa) so he doesn’t believe in the Rabb, like Fir ‘awn (Pharaoh), the Mu’attilah and the Dahriyyah. And as for the polytheist, then verily he believes in the Rabb but he commits shirk with Him and other than Him. As for the disbeliever then it can be due to heretical denial and it is also possible that he believes in the Rabb but he commits shirk with Him and other than Him. So between disbelief and polytheism there are generalities and specifics.
Disbelief Is Broader Than Shirk
There are specific and general differences between the disbelievers (al-kaafireen) and the polytheist (al-mushrikeen). Disbelief is broader than shirk, since every polytheist is a believer and not every disbeliever is a polytheist. The polytheist worships Allah’ and he also worships others. As for the disbeliever, then he denies the existence of Allah, the Majestic and Lofty, and he does not acknowledge Allah, nor does he acknowledge any of the religions. This is the disbeliever who is in denial. The polytheist acknowledges and believes, however, he worships Allah and others. Thus, he is a polytheist disbeliever, since every polytheist is a believer, and not every disbeliever is a polytheist, since the disbeliever could be an atheist in denial (of Allah’s existence).
Allah Accepts Repentance
Yes, if one repents then Allah accepts repentance, Allah accepts repentance from all of the Mudhnibeen [those who do dhamb (sins)] from amongst the Murtaddeen [apostates] and other than them. Allah subhanaanahu wa Ta’aalaa says: ”And verily I am indeed forgiving to him who repents, believes and does righteous good deeds…” (Q20 [TaHa]: 82) and He says, “say O my slaves who have transgressed against themselves, despair not of the mercy of Allah, verily Allah forgives all sins…” (Q35 [Zumar]: 53) Also Allah says: “Indeed those who disbelieve after them having faith and then went on increasing in their disbelief never will their repentance be accepted…” (Q 3[Al Imran]: 90).
Similarly, Allah Ta’aalaa says: “And whoever apostates from amongst you from his religion and dies as a believer, then his deeds will be lost in this life and in the hereafter…”(Q2[Baqarah]: 217). Meaning whoever apostates and does not repent then this is the increase in disbelief, his remaining upon disbelief and if he were to repent then Allah would accept his repentance; which indicates that if he died as a repentant Muslim then Allah would have accepted his repentance [taubah] and that is because Allah accepts the taubah from the apostate and other han him when he repents to Him; Azza wa jall.
[Answered by Shaykh saalih al-fawzaan;
one of the major Scholars of Saudi Arabia
and a prominent member of its
Fatwa Committee]