Leo XIII's Divinum illud munus (On the Holy Spirit)

We ought to pray to and invoke the Holy Spirit, for each one of us greatly needs His protection and His help. The more a man is deficient in wisdom, weak in strength, borne down with trouble, prone to sin, so ought he the more to fly to Him who is the never-ceasing fount of light, strength, consolation, and holiness. And chiefly that first requisite of man, the forgiveness of sins, must be sought for from Him: "It is the special character of the Holy Ghost that He is the Gift of the Father and the Son. Now the remission of all sins is given by the Holy Ghost as by the Gift of God. " Concerning this Spirit the words of the Liturgy are very explicit: "For He is the remission of all sins." How He should be invoked is clearly taught by the Church, who addresses Him in humble supplication, calling upon Him by the sweetest of names: "Come, Father of the poor! Come, Giver of gifts! Come, Light of our hearts! O. best of Consolers, sweet Guest of the soul, our refreshment!" She earnestly implores Him to wash, heal, water our minds and hearts, and to give to us who trust in Him "the merit of virtue, the acquirement of salvation, and joy everlasting." Nor can it be in any way doubted that He will listen to such prayer, since we read the words written by His own inspiration: "The Spirit Himself asketh for us with unspeakable groanings" (Rom. viii., 26). Lastly, we ought confidently and continually to beg of Him to illuminate us daily more and more with His light and inflame us with His charity: for, thus inspired with faith and love, we may press onward earnestly towards our eternal reward, since He "is the pledge of our inheritance" (Eph. i. 14).

Excerpted from Leo XIII's encylical letter on the unity of the Church, Divinum illud munus, 9 May, 1897.