Here is the footnote from the Aquinas Study Bible
2:27 It means,you need not go to false apostles and heretics to teach you the truth, for you have already learned it from the Apostles themselves, and that which they taught outwardly, the Holy Spirit must needs teach you within… Bellarmine, (de Verbo Dei, 3.3), who says, “You have no need for a Lutheran or Calvinist to teach you Christian doctrine, because you have been fully taught it by the teaching of the Church, and the aid of the Holy Spirit.” And St. Augustine thus writes. “I for my part have spoken to all. But they to whom that unction speaks not within, they whom the Holy Spirit teaches not, go away untaught. The outward teachings of a master are a kind of aid and warning, but He who teaches the heart has His seat in heaven. . . . One is your master, even Christ. Let Him speak to you within, when no one is present. For though some one is atyour side, yet there is no one inyour heart. Let there be no one inyour heart, let Christ be inyour heart, let His unction be inyour heart, lestyour heart be athirst in the desert, and have no fountains to water it. The Master who teaches is within, Christ teaches, His inspiration teaches. But where His inspiration and His unction are not, words echo in vain from without.” And so too St. Gregory, expounding these very words, says, “Unless the same spirit be in the heart of the hearer the words of the teacher are useless;” and he adds, “Do not ascribe to the teacher that which*you hear from his lips, for unless He who really teaches you be within, the tongue of the teacher labours outwardly in vain.” (Cornelius a Lapide)