to show that the law of works, which is divine and good, nevertheless does not help a person but kills if one lacks the gift of the Holy Spirit. Grace assists and aids in synthesizing law and grace, so that for Augustine letter/spirit becomes law/grace. The law causes flight from sin, and grace completes the journey. Grace, the gift of the person of the Spirit, is simply the law. Thus law ceases being an external thing concerning deeds and becomes internal, written on the heart when grasped in prayer.
Page 415