To begin, let us concede on two matters for the purpose of the argument. Let us suppose that there is some universally accepted standard of accepting evidence. Let us also assume that the Quran does indeed lack undeniable evidence of its divine origin. In modus tollen, the following argument is formed:

Premise 1: If the author of the Quran is God and God intended to furnish the Quran with undeniable evidence of its divinity, then we would find the Quran to be undeniably written by God.

Premise 2: We do not find undeniable evidence that the author of the Quran is God.

Conclusion: Therefore it is not true that God authored the Quran and intended to furnish the Quran with undeniable evidence of its divinity.

Let us define Premise 1 as P and Premise 2 as Q. In predicate calculus, the argument above can be written as:

P1: P → Q
P2: ¬Q
C: ∴ ¬P

Again, let us suppose ¬Q is true. The error does not arise in this premise, rather it arises in P → Q, due to a hidden subpremise. That is: “If God gave us evidence for a conclusion, then we would find the evidence undeniable. Let us again, define P’ as “God gave us evidence for a conclusion” and Q’ as “We found the evidence undeniable.”

P	Q	P’	Q’	P’ → Q’
T	T	F	F	T
T	T	F	T	T
F	F	T	F	F
F	F	T	T	T

It is important to note that P’ ⇒ Q’ ≡ (P’ ∧ Q’) ∨ (¬P’ ∧ ¬Q’) ∨ (¬P’ ∧ Q’), as seen in the second truth table below:

(P’ ∧ Q’)	(¬P’ ∧ Q’)	(¬P’ ∧ ¬Q’)	(P’ ∧ Q’) ∨ (¬P’ ∧ Q’) ∨ (¬P’ ∧ ¬Q’)
F	F	T	T
F	T	F	T
F	F	F	F
T	F	F	T

What we see here is that the logical fallacy in the argument is that the argument only considers the possibility of:

(P’ ∧ Q’) ∨ (¬P’ ∧ Q’) ∨ (¬P’ ∧ ¬Q’)

as if they are the only options present in this argument. This is not the case, as the argument does not account for the fourth option:

(P ∧ ¬Q)

Which corresponds to:

“God gave us evidence for a conclusion, and we did not find it undeniable.”

When God gives evidence for a conclusion, it can be to test the audience for a bias, as opposed to the sole purpose of convincing the audience.

It is obviously not feasible to believe that God would create undeniable proofs of Islam so that nobody would be able to reject Islam rather than just creating a species that unconditionally worship God without being convinced by these evidences. We know that, Islamically, God wishes to test us, hence, it does not make sense for God to provide undeniable evidence as opposed to sufficient evidence. Here, sufficient can be defined to mean evidence that would convince an unbiased person. So the Quran leaves room for unreasonable doubt:

But they are in doubt, amusing themselves. (44:9)
— Saheeh International

Nay, but they play in doubt. (44:9)
— M. Pickthall

Not reasonable doubt.