1 That divine office which Jesus Christ received from His Father for the welfare of mankind, and most perfectly fulfilled, had for its final object to put men in possession of the eternal life of glory, and proximately during the course of ages to secure to them the life of divine grace, which is destined eventually to blossom into the life of heaven. Wherefore, our Saviour never ceases to invite, with infinite affection, all men, of every race and tongue, into the bosom of His Church: "Come ye all to Me," "I am the Life," "I am the Good Shepherd." Nevertheless, according to His inscrutable counsels, He did not will to entirely complete and finish this office Himself on earth, but as He had received it from the Father, so He transmitted it for its completion to the Holy Ghost. It is consoling to recall those assurances which Christ gave to the body of His disciples a little before He left the earth: "It is expedient to you that I go: for if I go not, the Paraclete will not come to you: but if I go, I will send Him to you" (1 John xvi., 7). In these words He gave as the chief reason of His departure and His return to the Father, the advantage which would most certainly accrue to His followers from the coming of the Holy Ghost, and, at the same time, He made it clear that the Holy Ghost is equally sent by-and therefore proceeds from-Himself and the Father; that He would complete, in His office of Intercessor, Consoler, and Teacher, the work which Christ Himself had begun in His mortal life. For, in the redemption of the world, the completion of the work was by Divine Providence reserved to the manifold power of that Spirit, who, in the creation, "adorned the heavens" (Job xxvi., 13), and "filled the whole world" (Wisdom i., 7).
Source: Divinum Illud Munus (Vatican.va)