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SCRIPTURE COMMENTARY #499
"And I will pray the Father, and he shall give you another Comforter, that he may be with you for ever, even the Spirit of truth: whom the world cannot receive; for it beholdeth him not, neither knoweth him: ye know him; for he abideth with you, and shall be in you. But the Comforter, even the Holy Spirit, whom the Father will send in my name, he shall teach you all things, and bring to your remembrance all that I said unto you." (John 14:16-17; 26)
THE HOLY GHOST is the Third Person of the Blessed Trinity. Our Lord calls Him "another Paraclete" [Comforter], who would, in His own stead, enlighten, sanctify, strengthen and comfort the apostles and those who came after them, till the end of all things. This shows us that the Holy Ghost must be a Divine Person, distinct from the Father and from the Son. We have just read our Lord's words: The Father will send him in My Name;" and in another passage He speaks of "he Paraclete, the Spirit of Truth, whom I will send you from the Father." The Holy Ghost is, therefore, the Spirit of the Father and the Son, who proceeds from all eternity from the Father and the Son, and who in time was sent by the Father and the Son.
[From 'A Practical Commentary on Holy Scripture' by Bishop Knecht, D.D.]
(1899 Douay-Rheims Bible)