We have seen that divine grace is a contributing cause of faith. For example, there must be (a) grace2 given to move the will to desire to believe, and (b) prevenient "regenerative" grace1 to rid one of the vices that would interfere with accepting grace2 by fully embracing this desire. The soul must be healed and only then given faith.
What of justification? Barring a private special revelation (which some saints have on occasion received), public special revelation and its best interpretations are what we all go on. That's all the "external inducement" and evidence we have.
Now we need to believe that this revelation comes from the same God as the God of the philosophers. We need to believe what this God tells us about Himself that natural reason cannot of itself discover. And we need to attribute the firmness of our faith to God.
Regarding the last of these, it seems that the feeling of peaceful assurance in the truth of the articles of faith is the work of God. Without it, doubt could not be overcome. So, can't we say that God through both grace1 and grace2 helps to justify the belief to the faithful?
There is an emotional component here, what I called (borrowing from William James) in a previous post "lyrical enchantment" with the facts proposed as revelation. For example, I cannot prove my faith beyond doubt to a non-Christian despite being convinced of its truth. Nevertheless, I feel justified in it, while the non-Christian would not be.
This aspect of justification has been called "the inner witness of the Holy Spirit."
This is pretty much implied by St. Thomas, too:
Sacred Scripture, since it has no science above itself, can dispute with one who denies its principles only if the opponent admits some at least of the truths obtained through divine revelation;
thus we can argue with heretics from texts in Holy Writ, and against those who deny one article of faith, we can argue from another.
If our opponent believes nothing of divine revelation, there is no longer any means of proving the articles of faith by reasoning, but only of answering his objections -- if he has any -- against faith. (ST, I, 1, 8)
But if I can't prove the articles of faith to an anti-Christian opponent, what makes me so sure? How do I prove them to myself? Well, I can by means of personal mystical illumination = the grace of God = the witness of the Holy Spirit, as helping to both cause and justify faith.
Now in my book, I claim to solve the Gettier problem in epistemology. I do so by rejecting the definition of knowledge as justified true belief. Instead, I divide knowledge into two kinds: philosophical as true belief, and scientific as something which "sounds reasonable," i.e., seemingly well justified. I then show how this enriches our concept of knowledge and helps against various Gettier cases.
So, I have philosophical knowledge that Christ is Lord, because I believe it, and if it happens to be true.
But do I have scientific knowledge of the same? Why does Christianity sound reasonable to me?
(Note that an idea that sounds reasonable and so is known "scientifically" need not be true, and one need not believe it. E.g., the idea that the Twin Towers were brought down by controlled demolition has been defended with some vigor, so I do not despise it; yet it may well be false, and I withhold belief in any account of 9/11 including this one.)
Well, there is the (1) public evidence in the Scripture. But we have seen that it's insufficient. It has to be supplemented with something else, which is in this case (2) grace infused into the soul by God. The combined strength of the two (1') for Christianity (2') for me personally exceeds the influence of any other religious doctrine.
But what is the essence of this grace? William Lane Craig contrasts two possible "senses of divinity": his own (i) "deep assurance of salvation and reconciliation to God"; and Plantinga's (ii) "deep seated conviction that Christianity is true."
Given (i), the content of faith would have to be rationally compared and arrived at. You read the Old Testament, the New Testament, the Koran, whatever, and decide which is best or most attractive. Desire kindled by God helps to confirm belief in the most plausible doctrine as you see it by reason alone without a doubt.
If, however, you change your mind and switch from Christianity to Islam, then the Holy Spirit would appear to have to testify to the truth of Islam.
Unless, that is, one argues that the Spirit testifies only to Christianity and that an adherent of no other religion literally ever felt a deep assurance of salvation and reconciliation to God. (One interesting argument in favor of this proposal is that only Christianity acknowledges the Holy Spirit. It would seem strange for this person of the Trinity to testify to the truth of a faith or metaphysics that denies His very existence.) Otherwise, the Spirit seems useless for arbitrating between religions.
Given (ii), desire kindled by God inclines you to believe specifically Christianity. Even if you are presented with devastating objections to Christianity and are at a loss for answers, you may succeed at keeping the faith, because the Holy Spirit is "an intrinsic defeater-defeater."
This again implies that no non-Christian ever felt a deep seated conviction that his own faith is true, or at least that some may have felt it but were deluded. But then "deep seated conviction" is of little value in justification: how can I tell that I am not the deluded one?
This also seems to imply a certain abdication of responsibility on the part of the believer: the Holy Spirit "will provide" even if one's grasp of the Christian faith is outrageously wrong.
So, I don't know which is right. Perhaps this dilemma is an indication that speculative theology and specific doctrines are less important than how we live.