St Maximus the Confessor. Detail of a byzantine fresco by Manuel Panselinos (late 13th century) in the Protaton Church in Karyes, Mt. Athos.
The Holy Spirit is present unconditionally in all things, in that He embraces all things, provides for all, and vivifies the natural seeds within them. He is present in a specific way in all who are under the law, in that He shows them where they have broken the commandments and enlightens them about the promise given concerning Christ. In all who are Christians He is present also in yet another way in that He makes them sons of God. But in none is He fully present as the author of wisdom except in those who have understanding, and who by their holy way of life have made themselves fit to receive His indwelling and divinizing presence. For everyone who does not carry out the divine will, even though he is a believer, has a heart which, being a workshop of evil thoughts, lacks understanding, and a body which, being always entangled in the defilements of the passions, is mortgaged to sin. (First Century of Various Texts, 73, Philokalia, vol. 2, pp. 180-181)- St Maximos the Confessor (580-662)
Source: Mind in the Heart