Al-An'am: 19
He intended: "What witness is the greatest in testimony?" So he placed "thing" (shay’) in the position of "witness" (shahid) to exaggerate the generalization.
"Say, 'Allah is witness between me and you.'"
It is possible that the answer is complete at His saying: "Say, 'Allah,'" meaning: Allah is the greatest in testimony. Then, a new sentence begins: "Witness between me and you," meaning: He is a witness between me and you.
It is also possible that "Allah is witness between me and you" is the entire answer, as it indicates that when Allah—Mighty and Majestic is He—is the witness between him and them, then He is the greatest thing in testimony for him.
This is a conjunction referring to the pronoun of the addressees among the people of Mecca. That is: "That I may warn you thereby, and warn everyone whom the Qur'an reaches, among the Arabs and the non-Arabs."
It is said: "Among the Jinn and Mankind."
It is said: "Everyone whom it reaches until the Day of Resurrection."
From Sa'id ibn Jubayr: "Whoever the Qur'an reaches, it is as if he has seen Muhammad, may Allah bless him and grant him peace."
"Do you indeed testify..."
This is an affirmation for them, accompanied by denial and dismissal.
"Say, 'I do not testify [to what you testify].'"
"Those to whom We have given the Scripture know it as they know their own sons. Those who have lost themselves, so they do not believe. And who is more unjust than one who invents a lie about Allah or denies His verses? Indeed, the wrongdoers will not succeed."