39 It is plain, then, that all Christian virtues should flourish in the soul of the priest. Yet there are some virtues which in a very particular manner attach themselves to the priest as most befitting and necessary to him. Of these the first is piety, or godliness, according to the exhortation of the Apostle to his beloved Timothy: Exerce . . .teipsum ad pietatem , "exercise thyself unto godliness." Indeed the priest's relations with God are so intimate, so delicate and so frequent, that clearly they should ever be graced by the sweet odor of piety; if "godliness is profitable to all things," it is especially profitable to a right exercise of the priestly charge. Without piety the holiest practices, the most solemn rites of the sacred ministry, will be performed mechanically and out of habit; they will be devoid of spirit, unction and life. But remark, Venerable Brethren, the piety of which We speak is not that shallow and superficial piety which attracts but does not nourish, is busy but does not sanctify. We mean that solid piety which is not dependent upon changing mood or feeling. It is based upon principles of sound doctrine; it is ruled by staunch convictions; and so it resists the assaults and the illusions of temptation. This piety should primarily be directed towards God our Father in Heaven; yet it should be extended also to the Mother of God. The priest even more than the faithful should have devotion to Our Lady, for the relation of the priest to Christ is more deeply and truly like that which Mary bears to her Divine Son.
Source: Ad Catholici Sacerdotii (Vatican.va)