[...] He is able to search into all our thoughts, and penetrate into every motive of the heart.
Therefore, He pervades us…as light pervades a building, or as a sweet perfume the folds of some honourable robe; so that, in Scripture language, we are said to be in Him, and He in us.
It is plain that such an inhabitation brings the Christian into a state altogether new and marvellous, far above the possession of mere gifts, exalts him inconceivably in the scale of beings, and gives him a place and an office which he had not before.
In St. Peter’s forcible language, he becomes “partaker of the Divine Nature,” and has “power” or authority, as St. John says, “to become the son of God.”
Or, to use the words of St. Paul, “he is a new creation; old things are passed away, behold all things are become new.”
[...] This wonderful change from darkness to light, through the entrance of the Spirit into the soul, is called Regeneration, or the New Birth; a blessing which, before Christ’s coming, not even Prophets and righteous men possessed, but which is now conveyed to all men freely through the Sacrament of Baptism.
By…the coming of the Holy Ghost, all guilt and pollution are burned away as by fire, the devil is driven forth, sin…is forgiven, and the whole man is consecrated to God.
And this is the reason why He is called “the earnest” of that Saviour who died for us, and will one day give us the fulness of His own presence in heaven.
Hence, too, He is our “seal unto the day of redemption;” for as the potter moulds the clay, so He impresses the Divine image on us members of the household of God.
And His work may truly be called Regeneration; for though the original nature of the soul is not destroyed, yet its past transgressions are pardoned once and for ever, and its source of evil staunched and gradually dried up by the pervading health and purity which has set up its abode in it.
Instead of its own bitter waters, a spring of health and salvation is brought within it; not the mere streams of that fountain, “clear as crystal,” which is before the Throne of God , but, as our Lord says, “a well of water in him,” in a man’s heart, “springing up into everlasting life.”
Hence He elsewhere describes the heart as giving forth, not receiving, the streams of grace: “Out of his belly shall flow rivers of Living Water.” St. John adds, “this spake He of the Spirit” (John 4:14; 7:38, 39).
John Henry Cardinal Newman (1801-1890): Parochial and Plain Sermons, vol. 2, Sermon 19, The Indwelling Spirit.