Tafsir of Al-Mu'minoon 23:90

(Nay, We have brought them the truth) is a transition (idrab) from their statement: "This is nothing but the fables of the ancients."

The intended meaning of "the truth" is the promise of the Resurrection. It is also said: It refers to what includes that and Monotheism (Tawhid), and the context indicates this.

It has been recited as (bal ataytuhum / Nay, I have brought them) with the first-person pronoun. Ibn Abi Ishaq recited it as (bal ataytahum / Nay, you have brought them) with the second-person pronoun.

(And indeed, they are liars)

In their statement: "This is nothing but the fables of the ancients," or in that and their speech which contradicts Monotheism.