The Ruling on Swearing by a Religion Other Than Islam

We move on to another issue from the sister (A. N.) from Madinah al-Munawwarah. She says: My husband married another woman, and many problems arose between us. I swore that if he brought her into my house, I would be out of the religion of Islam unless I left the house. Then, to please my mother, I re-entered the house after having left it, and he brought her in. Am I sinful? And if there is an expiation, what is it?
Answer: If you intended by that that you would not stay with her, then you owe the expiation for an oath. But if you intended only to leave and then return, and you did leave, then you owe no expiation. However, you must repent from this statement; ... it is not permissible for a person to say this: "If I do such-and-such, I am out of Islam," or "I am out of Islam." The Prophet (peace and blessings be upon him) said: . Therefore, it is not permissible for a person to say: "If I do such-and-such, I am out of Islam," or "I am a Jew," or "I am a Christian," or "I am a disbeliever." This statement is not permissible. But if you said this statement: "By Allah, if he brings her into my house, I will leave," and you left, then you owe nothing in terms of expiation, because you did what you swore to. But you must repent from this oath, and from ... this statement. Repentance involves regret, not returning to such things, and seeking forgiveness. However, if you intended that you would leave and not return, meaning: you would not live with her, and that was your intention—your intention was that you would never live with her—and then you lived with her, then you owe the expiation for an oath. And you owe nothing but the expiation. The expiation for an oath is: feeding ten poor people, or clothing them, ten people, feeding them with a lunch or a dinner, or giving each person half a sa' of dates or rice, or clothing each person ......... or freeing a slave, if freeing a slave is facilitated ......, and whoever is unable to do the three—feeding, clothing, and freeing—fasts for three days. And you must repent from this statement. This statement is never permissible. Swearing to leave Islam is not permissible.