Suppose an opponent said: “We grant that the Speech of God is a necessary attribute, by virtue of which He has beginninglessly signified information. And we grant that Muhammed [ﷺ] is a man who God revealed some of this information to. But what is the proof that God did not signify lies, revealed those lies to Muhammed [ﷺ], and Muhammed [ﷺ] unknowingly delivered those lies to mankind?”
We respond: either:
- Lying is necessary for God, such that signifying truth is impossible for Him.
- Lying is possible for God, such that if He did signify a lie, He could have signified the truth instead of it.
- Lying is impossible for God, such that signifying truth is necessary for Him.
The first two options are impossible, so the third must be true.
As for the first, it is impossible because we have established that some of what God revealed to Muhammed ﷺ is true by rational necessity. Information like the emergence of the world, God’s own existence, and the falsehood of polytheism. And the truth of some of this revealed information, is proof that signifying truth is not impossible for God.
As for the second, it is impossible because it entails the contingency of Speech. As this would mean that the Speech is contingent upon an extrinsic specifier to select between God’s signifying either truth or falsehood, by virtue of this Speech’s existence. And you have already granted that Speech is a necessary attribute.
Thus, the third option is necessarily true by process of elimination. And it is rationally impossible for God to signify lies.