Is repentance from Lordship or atheism acceptable, or is it unforgivable Shirk (polytheism)?

A sincere repentance is accepted from its owner, no matter how great his sin, and the proof for this is the saying of Allah the Almighty: "And indeed, I am Forgiving to whoever repents, believes, and does righteousness, and then is rightly guided" (Taha: 82), and "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah . Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" (Az-Zumar: 53), and also His saying: "Except for those who repent, believe and do righteous work. For them Allah will change their evil deeds into good ones. And Allah is ever Forgiving and Merciful." (Al-Furqan: 70), and also: "But those who committed evil deeds and then repented after them and believed – indeed your Lord thereafter is Forgiving and Merciful." (Al-A'raf: 153). Therefore, it is advised to adhere to obedience to Allah, strengthen faith through beneficial knowledge, and make abundant supplications.