Does a Muslim fall into shirk (polytheism) or kufr (disbelief), and is his repentance accepted, if he says, "There is no god but so-and-so," while knowing that there is no god but Allah, but he said it in a state of lust and loss of reason?

Whoever utters this heinous expression, understanding its meaning, has committed disbelief (kufr). Such a person must offer sincere repentance, return to Islam by articulating the Two Testimonies of Faith, and feel remorse. It is recommended for them to perform ghusl (ritual bath), for Allah forgives all sins for those who repent, as He Almighty said: "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.'" And the Prophet, peace and blessings be upon him, said: "The one who repents from sin is like one who has no sin." They must also adhere steadfastly to Allah's law and increase in acts of obedience.