In the same article, Aquinas responds to this objection (it is worth pointing out that the question in view is whether the Law of the Gospel is a written law or not):
Further, the law of the Gospel is proper to those who are in the state of the New Testament. But the law that is instilled in the heart is common to those who are in the New Testament and to those who are in the Old Testament: for it is written (Wisdom 7:27) that Divine Wisdom "through nations conveyeth herself into holy souls, she maketh the friends of God and prophets." Therefore the New Law is not instilled in our hearts. [Objection 3]
St. Thomas replies:
No man ever had the grace of the Holy Ghost except through faith in Christ either explicit or implicit: and by faith in Christ man belongs to the New Testament. Consequently whoever had the law of grace instilled into them belonged to the New Testament. [ad 3; emphasis added]
Note that we receive the grace of the Holy Spirit through faith; but it's important to see that it is grace, and that it is not contingent upon the character of our faith. This is a significant and telling difference between the Catholic Gospel and the Protestant idea of "sola fide," by which it is understood that a man must have explicit faith in Christ [alone] for salvation.
But we are not saved by our faith; we are saved by the grace of God through Jesus Christ. To suggest that our faith must be explicit leaves no room for hope for those who – through no fault of their own – are unable to have explicit faith, such as the infant or the mentally infirm. Furthermore, the typical Protestant insists that he must retain this explicit faith to the end of his life; but this leaves no room for those who through infirmity lose the capacity for faith.