I would agree with you that faith is not mere mental assent, nor would I argue that faith is even primarily mental assent. St. James reminds us that “even the demons believe, and tremble”
But neither do I believe that faith is intra nos, on the contrary: faith is extra nos, it comes from outside of ourselves–it is something God gives, works, and strengthens. Faith is God’s work, not our own. It is precisely because faith comes by hearing and hearing by the word (Romans 10:17) that we cannot attribute faith to ourselves, but rather “it is the gift of God” (Ephesians 2:8). Further we read that it is not by our own will, but God’s will, that we have been born anew (John 1:13).
It is not automatic or robotic; but it is a supernatural working of the Holy Spirit to create in us what did not exist before: faith. Apart from the Holy Spirit, I cannot believe.
Now, as said already, I do not regard faith as mere nor primarily as mental assent; but to use the words of Martin Luther, “a bold trust in God’s grace”. That’s something the demons certainly do not (and cannot) have. But it is something which, by the power and work of the Holy Spirit, humans can have; not as the operating power of the human intellect, but as the supernatural work and gift of God.
This is why I am fully persuaded that even the infant and small child can believe. When infants and young children were brought before the Lord He rebuked His disciples for trying to prevent the little ones from coming to Him, “for to such as these belong the kingdom of God” (Luke 18:16). If it is impossible for an infant to have faith (because it requires a faculty of human intellectual or reasoning prowess), and if none can see the kingdom of God unless they are born again (John 3:3-5), then we exclude certain human beings from membership in God’s royal family, His Church, based upon the criterion of their own ability/inability.
I am not suggesting a TULIP-esque position wherein God has, by sovereign decree, decided who will and will not be saved; and thus the elect having been chosen will most certainly believe (whether they like it or not) and all others (whether by selection or by negation) are passed over and are unavoidably damned.
I am saying, however, that the power and working of God graciously condescending to meet us in Word and Sacrament means that the human heart is changed by the Holy Spirit, and where there was once emptiness, it becomes filled with faith, and in this the human person is indeed a passive recipient. What comes after, our response, the choice to follow or deny, to obey or disobey–all of that matters, for all the reasons Jesus gives in the Parable of the Sower. And that is where St. James’ words about the importance of our works and that “mere belief” doesn’t do us any good when even the demons believe and tremble. It is not “mere belief”, but it is faith alone through which we receive the gift of Christ’s own righteousness, so that all who trust in the Lord are not put to shame.
In the quoted passage from St. Justin’s Apology the blessed and holy martyr is arguing against the Pagan idea of fate. Pagans believed in fate, that the course of a person’s life, whether they were just or wicked, kings or slaves, was decided in some cosmic background, by the gods, written in the stars, etc. Justin very explicitly rejects this idea, unapologetically so. As we all should. Fate is non-Christian. Human beings are not fated to be good or evil, righteous or unrighteous, their status as kings or slaves or whatever is not written in the stars, woven by the threads of capricious “gods”. Human beings are created with the dignity of choice.
The doctrine of Total Inability is not a rejection of human dignity and volition; but a recognition that the problem of sin is so deep inside of us that it requires literal Divine Intervention to rescue us; and that Intervention is not found in the giving of a command. The Law cannot make us righteous, the Law which should give life, instead brings death. Not by a deficiency in the Law, but the deficiency in ourselves on account of sin (Romans 7:7-25). Which is also why the Apostle had aforementioned, quoting the Scriptures, said “There is no one that is righteous, not even one” “There is no one who seeks after God” (Romans 3:10-12).
It has nothing to do with cosmic (or divine) “fate” determining whether a person is good or evil; it is that all of us are fully and entirely sinful (Romans 3:23) and thus all of us need the Divine Intervention of the Gospel, since “God consigned all to disobedience in order that He might have mercy on all” (Romans 11:32).
So that through this Gospel the Holy Spirit works in us to create faith, and have become born again as new people; a new man, a new creation, in Christ; born of God, a child of adoption.
Some closing remarks from the holy and blessed doctor of grace, St. Augustine:
“And he says that a man is justified by faith and not by works, because faith itself is given, from which may be obtained other things which are specially characterized as works, in which a man may live righteously. For he himself also says, ‘By grace you are saved through faith; and this is not of yourselves; but it is the gift of God,’–that is to say, ‘And in saying “through faith,” even faith itself is not of yourselves, but is God’s gift.’ ‘Not of works,’ he says, ‘lest any man should be lifted up.’” - St. Augustine, On Predestination, I.12
“The spirit of grace, therefore, causes us to have faith, in order that through faith we may, on praying for it, obtain the ability to do what we are commanded. On this account the apostle himself constantly puts faith before the law; since we are not able to do what the law commands unless we obtain the strength to do it by the prayer of faith.” - St. Augustine, On Grace and Free Will, 28