The job of the Holy Spirit regarding faith seems to be to remove doubts regarding the articles of faith, as per St. Thomas’ understanding that the intellect can “assent to something, not through being sufficiently moved to this assent by its proper object, but through an act of choice, whereby it turns voluntarily to one side rather than to the other: and if this be accompanied by doubt or fear of the opposite side, there will be opinion, while, if there be certainty and no fear of the other side, there will be faith.” (ST, II-II, 1, 4) I mentioned the false religious beliefs of, say, Jews and Muslims, such as that Jesus is not God. Sometimes these beliefs are firmly and even fanatically held.
This bothered me somewhat, so I asked the Holy Spirit whether He ever acted to convince someone of a false belief, perhaps for reasons of overall “utilitarian” providence. He answered no, but I think it was presumptuous even to ask in light of 1 John 4:
Beloved, do not trust every spirit but test the spirits to see whether they belong to God, because many false prophets have gone out into the world. This is how you can know the Spirit of God: every spirit that acknowledges Jesus Christ come in the flesh belongs to God, and every spirit that does not acknowledge Jesus does not belong to God. This is the spirit of the antichrist that, as you heard, is to come, but in fact is already in the world. (1-3)
Deception is possible in spiritual phenomena and may be tested by its relation to Christian doctrine: those who fail to acknowledge Jesus Christ in the flesh are false prophets and belong to the antichrist.
Even though these false prophets are well received in the world, the Christian who belongs to God has a greater power in the truth.
So it seems rather that the Jews and Muslims, in regard precisely to their devotion to their corrupt religions, are victims of the influence of evil spirits. How unfortunate, especially because “the devil made me believe it” is hardly a valid excuse.
But how can a person honestly contemplating whether Christianity or Islam is true in order to choose between them be sure whether he is being prompted by the spirit of God or of the devil? Well, the passage continues: “You belong to God, children, and you have conquered them, for the one who is in you is greater than the one who is in the world.” (1 John 4:4) It may be that a genuinely honest inquiry, especially made through a pure heart, is guaranteed to lead one, at least eventually, to the truth.