Tafsir of Al-Anbiya' 21:54

He, peace be upon him, refuted them using the method of emphatic swearing, as he said: "You have certainly been, you and your forefathers—those whom you found to be likewise—in a manifest error."

"Manifest" means apparent and clear, such that its nature as an error is not hidden from any person of reason, as you and they rely upon no evidence, but rather upon followed whims and a obeyed devil. "You" [antum] serves as an emphasis for the attached pronoun in "you have been" [kuntum], which is necessary according to the Basran school of grammar to allow for the conjunction that follows such a pronoun.

The meaning of "you have been in error" is the absolute establishment and persistence of their state within it, not merely a past state that occurred before the time of the address, which would encompass both them and their forefathers. In the choice of "in error" [fi dalalin] over "erring" [dallin], there is an undeniable hyperbole regarding the depth of their error. The verse serves as evidence that falsehood does not become truth through the multitude of those who cling to it.