As for His words:

“And He made them testify concerning themselves: ‘Am I not your Lord?’ They said, ‘Yes.’”

In the Quran, a person’s testimony against himself means his admission. Whoever admits to an obligation incumbent upon him has thereby testified against himself. Allah Most High says:

“Stand firmly for justice as witnesses for Allah, even against yourselves, your parents, or close relatives.”

This is among the passages the jurists cite as evidence for accepting an admission. In the hadith of Mā‘iz ibn Mālik, it states: “When he had testified against himself four times, the Messenger of Allah, may Allah bless him and grant him peace, had him stoned”—meaning that he had confessed four times.

Another example is His statement, Most High:

“It is not for the polytheists to maintain Allah’s mosques while bearing witness against themselves to their disbelief.”

They acknowledged what constituted disbelief, and that was their testimony against themselves.

Allah Most High also says:

“O company of jinn and humankind, did messengers not come to you from among yourselves, relating My verses to you and warning you of the meeting of this Day of yours? They will say, ‘We bear witness against ourselves.’ The worldly life deluded them, and they will bear witness against themselves that they were disbelievers.”

Their testimony against themselves is their admission; this, then, is what testimony against oneself means.