Depravity and mysteries above nature both demand the Spirit's work for faith
Human crookedness, plus the darkness sin has piled on, forces this need for God's Spirit in the soul. Scripture is full of matter that fights our warped loves. The flesh-bound person therefore recoils and turns away — and that recoil blocks true acknowledgment and right judgment, because we all judge by the grain of our affections.
Sin also darkens the mind so thoroughly that even among ordinary natural things we keep hallucinating and falling into error. How much less, then, can we be sure of what stands above nature? So the Holy Spirit must drive out the mind's native night, and clear the smoke by which wicked desires cloud the intellect and stop it from seeing truth.
Nor is depravity the only reason. The very character of the doctrine Scripture teaches also requires this Spirit work if faith is to be born in us. Scripture holds mysteries that outrun not only fallen nature but human nature taken by itself, even without sin. Nature cannot invent them, think them up, or even understand and take them in. So the Spirit must heal minds sin has blinded, and must also lift the intellect well above its natural level, until it can rise to things that great.
Latin
IV. Porro istam Spiritus Dei in animis nostris operationem necessariam reddit ingenii humani pravitas, et tenebrae quas peccatum accersivit. Etenim Scriptura sacra multa continet, quae pravis et distortis nostris affectibus repugnant. Unde fit ut homo carnalis ab illa abhorreat, et eam aversetur, quod impedit ne ejus veritatem agnoscat, et rectum de ea judicium ferat: quandoquidem prout quisque affectus est, ita judicat. Deinde mens hominis in universum ita per peccatum obscurata est, ut, etiam in naturalibus, non possit non hallucinari, et saepissime falli. Ac proinde multo minus quae supra naturam sunt certo agnoscere et percipere potest. Ad hoc igitur necesse est, ut Spiritus sanctus mentis tenebras nativas dispellat, et fumos illos, quibus affectus pravi intellectum obscurant, et praepediunt ne possit veritatem cernere. V. Imo hanc Spiritus operationem ad fidem animis nostris ingenerandam necessario exigit, non tantum naturae nostrae depravatio, et tenebrae ex peccato ortae, sed genius ipse doctrinae in Scriptura traditae. Siquidem multa mysteria continet, quae non tantum naturam corruptam, sed naturam humanam in se, etiam citra peccatum, consideratam, omnino superant: ita ut ab ea, non solum non inveniri et excogitari, sed nec intelligi et percipi possint. Ideoque necesse est, ut Spiritus sanctus non tantum mentes nostras peccato caecas sanet, sed etiam ut intellectum nostrum non parum supra statum naturalem extollat, ut ad res tantas possit assurgere.