St. Thomas Aquinas, Summa theologiae II-II.8.4.Praet 1, trans. FEDP (Shapcote). Ad 1: "Some who have sanctifying grace may suffer from dullness of mind with regard to things that are not necessary for salvation; but with regard to those that are necessary for salvation, they are sufficiently instructed by the Holy Ghost, according to 1 John 2:27, His unction teacheth you of all things."