Grace is the very energies of God himself. The Orthodox Church believes that through the Trinitarian ministry of the Holy Spirit these energies are mediated to mankind. That is, how God acts in forgiving and spiritual healing. Grace is the working of God himself, not a created substance of any kind that can be treated like a commodity.
Human will and God's grace
It is the teaching of the Church that the grace of God invites all but compels none. "Behold, I stand at the door and knock; if anyone hears my voice and opens the door, I will come in" (Revelation 3:20). One cannot achieve full fellowship with God (theosis) without God's help, yet one must also play one's own part to the common work with God. Although what God does is of immeasurably greater importance, divine grace and human will are equally necessary.
The sacraments are seen as a "means of grace" because God works through his Church, not just because specific legalistic rules are followed.
The Church emphasizes the role of the Holy Spirit in Christian life, and has maintained ascetical disciplines such as fasting and almsgiving, not as a way to make penance for past sins nor to build up grace, but as a means of spiritual discipline, to help reduce sin in the future. There is no doctrine of purgatory or "treasury of surplus merit."
"The roof of any house stands upon the foundations and the rest of the structure. The foundations themselves are laid in order to carry the roof. This is both useful and necessary, for the roof cannot stand without the foundations and the foundations are absolutely useless without the roof—no help to any living creature. In the same way the grace of God is preserved by the practice of the commandments, and the observance of these commandments is laid down like foundations through the gift of God. The grace of the Spirit cannot remain with us without the practice of the commandments, but the practice of the commandments is of no help or advantage to us without the grace of God."