—or by any of His names. Allah the Exalted says, “To Allah belong the Most Beautiful Names, so call upon Him by them” (Q 7:180), and He says, “Whatever name you invoke, to Him belong the Most Beautiful Names” (Q 17:110). Hence, if you swear by al-Raḥmān (the All-Merciful) or al-Raḥīm (the Bestower of Mercy) or al-Samīʿ (the All-Hearing), or by any other of the divine names, the oath is lawful.
Arabic possesses three particles that introduce an oath: the waw, the bāʾ, and the tāʾ. With the waw one says, “Wa-Allāh, I will surely do such-and-such”; with the bāʾ, “Billāh, I will surely do such-and-such”; and with the tāʾ, “Ta-Allāh, I will surely do such-and-such.” Allah says, “They swear by Allah their most solemn oaths” (Q 16:38) and “They swear by Allah to you in order to please you” (Q 9:62); He also says, “By Allah, you nearly ruined me” (Q 37:56), and, “No, by your Lord, they will not believe…” (Q 4:65). These, then, are the letters of oath.
Swearing by anything other than Allah is disbelief (kufr) or polytheism (shirk). It may amount to major disbelief or minor disbelief, just as it may be major or minor shirk. If the swearer believes that the object of his oath possesses a greatness equal to that of Allah, his act is major shirk. If he believes it has a lesser greatness—yet still renders it venerated—this is minor shirk, a pathway that leads to the greater. In the Days of Ignorance people customarily swore by their forefathers, so the Prophet ﷺ forbade it and said, “Do not swear by your fathers,” that is, nor by your brothers, grandfathers, or leaders. He mentioned fathers specifically because that was their habit. “Whoever must swear,” he said, “let him swear by Allah, or else remain silent”—meaning: either swear by Allah or do not swear at all; but do not swear by other than Allah.
Included in the prohibition is swearing by the Prophet Muḥammad ﷺ, the noblest and foremost of mankind. If you were to say, “By the Prophet Muḥammad,” you would be guilty of shirk or kufr. The same applies to swearing by Gabriel, Michael, Isrāfīl, Mālik the keeper of Hell, or anyone else; likewise swearing by the sun, moon, night, or day. All of that is shirk—major or minor according to the distinction just made.
It is, however, permissible to swear by an attribute of Allah, such as, “By Allah’s might I will do it,” or, “By Allah’s wisdom I will do such-and-such.” No harm attends this. But, as stated, swearing by anything other than Allah constitutes disbelief or shirk, whether major or minor.
The author then cited the hadith concerning one who says, “I repudiate the religion of Islam if such-and-such be the case.” A person is not allowed to utter such words. If he does so and is lying, he becomes—as he claimed—outside Islam, Allah forbid. If he is truthful, he will not return to Islam unscathed; sin or even outright disbelief will inevitably attach to him. The same ruling applies to saying, “He is a Jew if such-and-such happens,” or, “He is a Christian if such-and-such happens.” This is prohibited: if you are lying, you become what you have named—Jew or Christian; if you are truthful, you will not return to Islam unharmed.
By way of illustration: a man says, “So-and-so arrived today from travel.” His companion replies, “No, he did not.” The first insists, “If he has not arrived, then I am a Jew.” Should it turn out that the traveller has in fact not arrived, the oath-taker was lying and so becomes a Jew by his own statement. If, however, the traveller had arrived and the man’s claim was true, he still would not return to Islam safely, exactly as the Prophet ﷺ warned.
In short, when you need to swear an oath, swear by Allah—using any of His names or attributes. Someone might ask, “Did not Allah Himself swear by created things?—‘By the sun and its brightness’ (Q 91:1); ‘By the night as it envelops’ (Q 92:1); ‘By the sky and Him who built it’ (Q 91:5).” We reply: Allah may swear by whatever of His creation He wills. When He swears by something, it proclaims His own majesty, for the greatness of a created being points to the greatness of its Creator. Allah does not swear except by what is truly mighty, and the might of any creature derives from the might of the Creator. Thus Allah may swear by whatever He chooses, and no one may set limits upon Him—He does whatever He wills.
Another question: we hear some people say, “I swear by the āyāt of Allah.” Is that an oath by other than Allah, and therefore disbelief or shirk? The answer depends on what they mean by “the āyāt of Allah.” If they intend the sun, moon, night, day, and the like—things Allah has called “signs” (āyāt)—then it is an oath by other than Allah and constitutes shirk or kufr, for Allah says, “And of His signs are the night and the day, and the sun and the moon” (Q 41:37). But if they mean by “the āyāt of Allah” the Qur’an—whose verses are indeed the āyāt of Allah—then there is no shirk, because the Noble Qur’an is the speech of Allah, and His speech is one of His attributes. When someone says, “I swear by the āyāt of Allah, meaning the Qur’an,” the oath is sound and blame-free.
In my estimation, when the common folk use this expression, they usually mean the Qur’an; in that case it is not forbidden. However, if they intend by it the created signs—the sun, moon, stars, night, day, and the like—then it is shirk or kufr. And Allah is the One who grants success.