The person who puts to death by the spirit the deeds of our sinful nature will live, says the Apostle. This is most surprising since one who has the Spirit of God becomes a child of God. So true is it that he is a child of God that he received not a spirit that enslaves but the Spirit that makes us sons. So much so that the holy Spirit bears witness to our spirit that we are sons of God. This is the witness of the Holy Spirit: he cries out in our hearts, Abba, Father, as we read in the letter to the Galatians.
From a letter by Saint Ambrose, bishop