It is important to understand correctly in what true rest or peace of the soul consists. There is a rest which is more so in appearance than reality; just as there is a semblance, a counterfeit of humility, of benevolence, and of other Christian graces. There are some persons whose apparent rest is to be ascribed to natural inertness or stupidity, and not to the sanctified adjustment of their powers. The true rest, however, is not to be regarded as identical with inaction.
The rest of the soul, in the highest spiritual sense of the terms, is that state of the soul, whether it be in repose or in action, which is in harmony with God. There is only one right position of the soul. All others must necessarily be wrong. And that position is one where the creature is brought into perfect adjustment with the Creator, by deriving its perceptions from God, by merging its affections in God's affections, and by harmonizing its will with God's will. In such a state of the soul there must necessarily be rest, if God has rest.
— edited from A Treatise on Divine Union (1851) Part 8, Chapter 1.