What you mentioned about the person
saying: "Damn your Lord's Religion," this takes them out of Islam. It is obligatory that they should be advised and guided with wisdom, good advice, and the best arguments, so Allah may guide them and they will stop saying such things in the future. They should be advised to perform Tawbah (repentance to Allah) for what has happened in the past, as Tawbah, if accepted, would result in the forgiveness of the sins they committed. Allah (Exalted be He) says:
Say: "O ‘Ibâdî (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allâh: verily, Allâh forgives all sins. Truly He is Oft-Forgiving, Most Merciful.
Scholars have agreed by Ijma` (consensus) that this Ayah (Qur'anic verse) is concerned with those who perform Tawbah. Allah (Exalted be He) also says:
And verily, I am indeed forgiving to him who repents, believes (in My Oneness, and associates none in worship with Me) and does righteous good deeds, and then remains constant in doing them, (till his death).
There are many evidences in the Qur'an and Sunnah on the legitimacy of Tawbah.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.