37 It would be a grave error fraught with many dangers should the priest, carried away by false zeal, neglect his own sanctification, and become over immersed in the external works, however holy, of the priestly ministry. Thereby, he would run a double risk. In the first place he endangers his own salvation, as the great Apostle of the Gentiles feared for himself: "But I chastise my body, and bring it into subjection: lest perhaps, when I have preached to others, I myself should become a castaway." In the second place he might lose, if not divine grace, certainly that unction of the Holy Spirit which gives such a marvelous force and efficacy to the external apostolate.
Source: Ad Catholici Sacerdotii (Vatican.va)