Can the Holy Spirit ever lie? (Actually, like Mark Twain and unlike George Washington, He can lie; the proper question is, does He?), Well, it is at the very least obvious that He can withhold truth, especially since the content of the divine wisdom is infinite and telling any truth is grace, a free gift. God allows us to be deceived, as Descartes famously pointed out: “If, however, it is contrary to His goodness to have made me such that I constantly deceive myself, it would also appear to be contrary to His goodness to permit me to be sometimes deceived, and nevertheless I cannot doubt that He does permit this.” (Meditations, I) The Holy Spirit is under no general obligation to remedy that.
I can concede that God may lead people to different faiths for His own reasons. These reasons may depend on what opportunities God actually detects to improve us, on which faith will most benefit each particular person, on what’s going on with the angels and saints in heaven (or less pleasant places “up there”), on overall “utilitarian” providence, and so on. Can we expect the Holy Spirit to testify truthfully?
Regarding the commandments, God did murder a bunch of people both directly and indirectly in the Old Testament. Could He sometimes bear false witness, too? Not about things above human nature, such as the Christian articles of faith, or “official” prophecies about distant future, as we are helpless regarding them without Him. But regarding scientific things, I can see how He might mislead someone who does not deserve the truth, such as explicitly to make him more self-reliant. If a scientist is praying to God instead of conducting his experiments properly, I can see God getting annoyed and lying to the scientist in order to cause him to fail and thereby set him straight. There are the sins of superstition and temptation of God to remedy which God might lie. Or asking for a revelation of some supernatural truth, perhaps like the specifics of the afterlife or future events, in prayer that might not be our business to inquire of, may elicit a lie to impart the crucial lesson unto the believer to focus instead on his own salvation.
Or consider a question, “God, will I be saved?” This is a tricky one. For one, answering “no” might fill the person with despair, leading to self-fulfillment of the prophecy. And answering “yes” may fill him on the contrary with presumption, if he imagines that he is guaranteed heaven and immune from sin, also hindering salvation. If God does reply, it may well be a lie in order to prevent these perverse consequences. There are things in this life that are best not to know, and God, realizing this, might mislead you.
Or what if telling a person a divine secret will in the long run hurt his happiness or salvation? Since happiness is our last end, I think God would rather, foreseeing all the consequences of His own actions, tell a saving lie than a condemning truth.
Or what if a person is asking to be enlightened idly out of some bored curiosity? It does not belong to a man to bother God with pointless questions. God would not necessarily lie, but He definitely need not reveal anything He does not feel like revealing. I can see Him dismissing the question with an irritated “Yeah, sure, whatever.”
So then: There is truth in Islam, and if God helps to impress on a man a deep seated conviction that Islam is true, then He does so by testifying to the true aspects of Islam while abstaining from commenting on its false aspects. If this testimony thereby causes the man to accept Islam as a whole, both the truth and falsehood in it, then the false beliefs are no skin off the Holy Spirit’s nose. It’s not His fault the man was too enthusiastic and ultimately by his own choice and discernment swallowed the good along with the bad.
In that case, Craig is right that to the extent that the Holy Spirit continues to testify to a Muslim true believer, presumably it’s to turn him toward a better faith, such as indeed Christianity which at least teaches that the Holy Spirit exists!
And what of different Christian theologies? People have all sorts of wrong ideas about God or angels or the afterlife. God is not hurtling lightning bolts at them immediately to correct them all the time, though He may manifest His influence as He chooses.
This does suggest that even true sincere religious believers can be wrong about many particulars and even some fundamental things.
So, I think the witness of the Holy Spirit regarding divine things above our nature is always true and would be “an intrinsic defeater-defeater,” if one always listened to it carefully enough, or such as to identify His voice perfectly from all other considerations, or such as to act virtuously by fully accepting His grace. The flaw in not in God but in ourselves, and life is too complicated for knock-down arguments like this.
Moreover, my justification for Christianity is not weakened because Muslims are also well-justified in Islam. So, perhaps the deep assurance / conviction do justify a belief to each believer, but the belief itself need not be true or at least fully true. Again, however, regarding supernatural things, God never lies in the foregoing sense. As per the distinction I’ve drawn, both a Christian and Muslim true believer would have scientific knowledge of the best religion; but only one of them would have philosophical knowledge of it.