When does shirk in words become major kufr?

Question:
When does shirk in words become major kufr?

Answer:
When one calls upon other than Allah.
Q: If he says it while considering it permissible?
A: Even if he did not consider it permissible, if he calls upon other than Allah, it is major shirk, even if he says: I did not consider it permissible.
Q: I mean specifically in the context of swearing oaths?
A: Swearing by other than Allah is minor kufr, unless one believes that the one by whom he swears knows the unseen and that He intervenes in the universe; in that case, it is major kufr from the perspective of creed. However, if he swears by other than Allah because he considers it exalted, like an angel or a prophet or the like, then it is minor shirk.
Q: A phrase by some commentators says: "Shirk in words may be major kufr that expels one from the religion if a person considers it permissible, or belittles it, or mocks it"?
A: This is correct: [Surah al-Tawbah: 65, 66]. If one mocks the religion, or the Messenger, or Allah, it becomes major kufr. Or if one swears by other than Allah believing that the one by whom he swears intervenes in the universe, or that it benefits and harms, or that it knows the unseen; it becomes major kufr due to the creed. However, if it slips from his tongue without intention, like what slipped from the tongues of the Companions in the early days of Islam: "By your father," "No, by your father," "No, so-and-so"; this falls under the category of minor shirk.