Grace is most assuredly the primary vehicle and the starting point: Without it, man cannot assent to the supernatural truths of revelation.
The fact that Protestants can be deluded is as irrelevant to that article of faith, as that men can refuse it.
It is the starting point, but not the vehicle.
The Council of Trent (Sess. VI, cap. vi, and can. xii) decrees that not the fiduciary faith, but
a real mental act of faith, consisting of a firm belief in all revealed truths makes up the faith of justification and the "beginning, foundation, and source" (loc. cit., cap. viii) of justification.
“The Synod furthermore declares, that in adults, the beginning of the said Justification is to be derived from the prevenient grace of God, through Jesus Christ, that is to say, from His vocation, whereby, without any merits existing on their parts, they are called; that so they, who by sins were alienated from God, may be disposed through His quickening and assisting grace, to convert themselves to their own justification, by freely assenting to and co-operating with that said grace: in such sort that, while God touches the heart of man by the illumination of the Holy Ghost, neither is man himself utterly without doing anything while he receives that inspiration, forasmuch as he is also able to reject it; yet is he not able, by his own free will, without the grace of God, to move himself unto justice in His sight. Whence, when it is said in the sacred writings: Turn ye to me, and I will turn to you, we are admonished of our liberty; and when we answer; Convert us, O Lord, to thee, and we shall be converted, we confess that we are prevented by the grace of God…Now they (adults) are disposed unto the said justice, when, excited and assisted by divine grace, conceiving faith by hearing, they are freely moved towards God, believing those things to be true which God has revealed and promised…”
The fact that people can be deluded in matters of faith or outright reject Grace is known to man by faith which is possible only by his mind and ability to reason which is the most relevant part of this discussion.