Estius: charity perfects faith so it merits; one habit is not another's subject
Estius: charity is not faith's essential form, and one habit is not another's subject.
Rather, through charity faith is perfected and formed so as to be meritorious and to profit to eternal salvation — as charity is called the form of all other virtues.
Latin
V. Sic Scholae Romanae mentem explicat Gul. Estius: Non enim, inquit, charitas forma fidei dicitur, quasi ad essentiam fidei pertinens, aut quia fidem proprie informet, cum unus habitus non sit subjectum alterius. Sed hactenus, quia per charitatem aliud, fides perficitur & formatur, ut meritoria sit, & ad aeternam salutem proficiat, quomodo & aliarum omnium virtutum forma charitas appellatur. Ad Distinct. 23. lib. 3. Sententiarum, paragr. 3.