1 You remember, brethren, that yesterday's lesson was brought to a close at this point, that “you have no need that any man teach you, but the unction itself teaches you concerning all things.” Now this, as I am sure ye remember, we so expounded to you, that we who from without speak to your ears, are as workmen applying culture from without to a tree, but we cannot give the increase nor form the fruits: but only He that created and redeemed and called you, He, dwelling in you by faith and the Spirit, must speak to you within, else vain is all our noise of words.
Whence does this appear? From this: that while many hear, not all are persuaded of that which is said, but only they to whom God speaks within. Now they to whom He speaks within, are those who give place to Him: and those give place to God, who “give not place to the devil.” For the devil wishes to inhabit the hearts of men, and speak there the things which are able to seduce. But what says the Lord Jesus? “The prince of this world is cast out.” Whence cast? Out of heaven and earth?
Out of the fabric of the world? Nay, but out of the hearts of the believing. The invader being cast out, let the Redeemer dwell within: because the same redeemed, who created. And the devil now assaults from without, not conquers Him that has possession within. And he assaults from without, by casting in various temptations: but that person consents not thereto, to whom God speaks within, and the unction of which you have heard.
Source: Homilies on the First Epistle of John (New Advent)