What is the ruling on deliberately swearing falsely by Allah?

It is established in Islamic law that al-yamin al-ghamus (a false oath) is an oath in which a person deliberately lies to affirm or deny something, whether in relation to the past or the present. It is called ghamus (immersed) because it plunges the person who makes it into sin, as he knowingly swears falsely.

Such an oath constitutes a sin, based on the words of Allah Almighty, “Allah does not hold you accountable for what is unintentional in your oaths, but He holds you accountable for what your hearts have earned” (Quran, 2:225). The Prophet (peace and blessings be upon him) also said, “The major sins are: associating partners with Allah Most High, undutifulness to parents, taking a life, and false oaths” (recorded by Bukhari and Ahmad). The Prophet (peace and blessings be upon him) also said, “A false oath leaves homes barren” (reported by al-Bayhaqi in Al-Sunan al-Ṣughra). The expression “leaves homes barren” figuratively refers to the loss of prosperity and the poverty that may result from such an oath. A deliberate false oath requires repentance and seeking Allah’s forgiveness.

If the oath referred to in the question involved deliberately lying to affirm or deny something, then it is yamin al-ghamus, the ruling of which was explained above. However, if the person taking the oath genuinely believed that his statement was true, then it falls under laghw (an unintentional oath). There is no sin for such an oath, based on the words of Allah Almighty Who says, “Allah does not hold you accountable for what is unintentional in your oaths”
(Quran, 2:225).

And Allah Almighty knows best.