Faith resolves into the Spirit's impulse; faith's light is that by which we believe
Therefore we state that the last thing into which our faith finally resolves — its principal support — is that impulse and instinct of the Holy Spirit described in thesis 9. By that name we mean the Spirit's action and efficiency: he immediately moves us to receive divine revelation and impresses a sure persuasion of it on our minds.
A comparison with scientific assent helps. What demonstrated conclusions are in the sciences, the dogmas of faith and the other things God's Word teaches are here. What the middle term of a demonstration is to its conclusion, the authority of God speaking and revealing is to the things proposed for belief. As we immediately grasp the middle term and the force of a demonstration by reason's natural, inborn light, so we immediately believe and perceive the revelation given to the Prophets and Apostles through the Spirit's inward instinct and testimony — what the Scholastics call the light of faith.
And as reason's light is not something we come to know by reasoning, but that by which we know and reason, so this testimony of the Holy Spirit is not a further thing believed, nor an argument that moves us to believe. It is only that by which we believe — the action that impresses faith on the mind. Every believer who elicits an act of faith need not know and understand that action by reflexive cognition, any more than a country laborer must think about intelligence and the power of reasoning while he understands and reasons.
Latin
XXXIV. Quapropter ultimum illud in quod fides nostra tandem resolvitur, et praecipuum illius firmamentum, statuimus esse illum Spiritus sancti impulsum, et instinctum qui a nobis thesi 9. descriptus fuit. Quo nomine designamus Spiritus sancti actionem, et efficientiam, qua nos ad divinam revelationem suscipiendam immediate movet, et certam de ea persuasionem mentibus nostris imprimit. Ad cujus rei illustrationem potest aliquid conferre, si comparatio fiat inter assensum fidei, et assensum scientiarum. Quod ergo sunt in scientiis conclusiones demonstratae, hic sunt fidei dogmata, et alia quae docet et tradit Dei verbum. Quod est medium demonstrationis, conclusionis respectu, istud est Dei dicentis et revelantis authoritas, respectu eorum quae credenda proponuntur. Ut autem medium ipsum, et demonstrationis vim immediate percipimus, naturali et infito rationis lumine: ita divinam revelationem, factam Prophetis atque Apostolis, immediate credimus et percipimus, per internum Spiritus Dei instinctum et testimonium, quod fidei lumen Scholastici vocant. Atque ut rationis lumen non est illud quod ratiocinando percipimus et cognoscimus, sed illud quo cognoscimus et ratiocinamur: Ita istud Spiritus sancti testimonium non est amplius aliquid quod creditur, nec argumentum quod ad credendum moveat, sed illud tantum quo creditur, et actio qua fides menti nostrae imprimitur: quam non necesse est ut omnis fidelis, qui fidei actum elicit, cognoscat et intelligat cognitione reflexa: Non magis quam necesse est rusticum aliquem cogitare de intelligentia et vi ratiocinandi, quum intelligit et ratiocinatur.