What is the ruling on one who falsely swore that Islam was cursed in order for the aggressors to receive a harsher punishment, knowing that the cursing did not occur, and he is now remorseful for his action?

The action of this man, who swore a false oath to harm an innocent person, is an ugly deed and an injustice. This is forbidden in Islamic law, even if the wronged person is a Christian. This false oath is an injustice, a distortion of religion, and a hindrance from the path of Allah. He must repent to Allah with sincere repentance. Regarding the necessity of expiation for this oath, there is a difference of opinion among scholars. The majority view is that its sin is too great to be expiated. However, if he expiates as a precaution, that is good. He must also strive to remove the injustice from the wronged person.