Is concealing a past illicit relationship from a suitor, after having sworn on the Quran, a sin that warrants not being forgiven?

First: The suitor or husband should not ask about his wife's past. Her righteousness is sufficient. It is wrong to compel her to expose herself, and if she tells the truth, it opens the door to doubt and suspicion. Second: The wife or fiancée is not obliged to disclose what Allah has concealed. Rather, she must conceal it, based on the Prophet's (peace and blessings of Allah be upon him) saying: "Avoid these abominations that Allah Almighty has forbidden. Whoever commits one of them, let him conceal himself with the concealment of Allah Almighty." And his (peace and blessings of Allah be upon him) saying: "Allah does not conceal a servant in this world except that He conceals him on the Day of Resurrection." If the suitor or husband insists on asking, she may use equivocation (Tawriya). Some scholars have even permitted her to lie in such a situation. Third: If the suitor says to his fiancée: "You will be unlawful to me after marriage if you conceal anything from me," and she then conceals something, she is neither divorced nor is it considered Dhihar (a form of divorce by likening one's wife to a forbidden relative); because divorce and Dhihar only occur after marriage.