Augustine of Hippo

And this Holy Spirit is in Scripture especially spoken of by the number seven, whether in Isaiah or in the Apocalypse, where the seven Spirits of God are most directly mentioned, on account of the sevenfold operation of one and the self-same Spirit. And this sevenfold operation is mentioned in Isaiah: "the Spirit of wisdom and understanding, of counsel and fortitude, of knowledge and piety, the Spirit of the fear of God rested upon him." This fear of the Lord should be understood as pure, enduring forever.