The One whom the Father has sent into our hearts, the Spirit of his Son, is truly God. Consubstantial with the Father and the Son, the Spirit is inseparable from them, in both the inner life of the Trinity and his gift of love for the world. In adoring the Holy Trinity, life-giving, consubstantial, and indivisible, the Church's faith also professes the distinction of persons. When the Father sends his Word, he always sends his Breath. In their joint mission, the Son and the Holy Spirit are distinct but inseparable. To be sure, it is Christ who is seen, the visible image of the invisible God, but it is the Spirit who reveals him.      #689  Catechism of the Catholic Church.

The doctrine of the Holy Spirit is without doubt the most important of all the Church's teachings because, if we do not know and love the Holy Spirit, we cannot possibly understand the other great truths of our Holy Religion. Without the Holy Spirit we are blind!

Not only is this doctrine the most important, it is the most wonderful, the most consoling, the most sublime of all doctrines, for with the Holy Spirit we can do all things easily and well. He is the  Spirit of Love, of Peace, of Joy, the Spirit of Divine Consolation. He is the Light of Our Souls and the Strength of Our Wills. Yet this doctrine is little understood by a great number of Christians. Some have a vague knowledge of the Holy Spirit, but very few indeed have a real grasp of all the Holy Spirit has done for them and is most ready to do if only they allow Him, but many  rarely or ever think of the Holy Spirit!

"How extraordinary," Cardinal Manning exclaims, "it is that Christians know so little about the Holy Spirit though He is the Author of our sanctification, the Giver of all Joys and Consolations!" His Eminence was in his youth a sincere Protestant, he became, with the help of the Holy Spirit, a fervent Catholic. Under the same divine guidance, he became a priest, a bishop and finally a cardinal. He ever cherished a great devotion to the Holy Spirit and solved all his doubts and difficulties by praying to the Holy Spirit. When called on to make any important decision, he first of all bent his head in silent prayer. If the problems were graver, he devoted more time and fervor in asking for guidance. Thanks to the graces he thus received, he was able to not only to attain high personal  sanctity but to render great services to the Catholic Church in England. He wrote two beautiful books on the Holy Spirit.