33 Let us hear from our Lord's own words what is the work of the Holy Ghost within us. He says, I have yet many things to say unto you, but you cannot bear them now. For it is expedient for you that I go: if I go I will send you the Advocate. And again, I will ask the Father and He shall send you another Advocate, that He may be with you for ever, even the Spirit of truth. He shall guide you into all truth, for He shall not speak from Himself, but whatsoever things He shall hear He shall speak, and He shall declare unto you the things that are to come. He shall glorify Me, for He shall take of Mine. These words were spoken to show how multitudes should enter the kingdom of heaven; they contain an assurance of the goodwill of the Giver, and of the mode and terms of the Gift. They tell how, because our feeble minds cannot comprehend the Father or the Son, our faith which finds God's incarnation hard of credence shall be illumined by the gift of the Holy Ghost, the Bond of union and the Source of light.
Source: On the Trinity (New Advent)