E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Heart of Grace (To take).
To pluck up courage; not to be disheartened or down-hearted. This expression is based on the promise, My grace is sufficient for thee (2 Cor. xii. 9); by this grace St. Paul says, When I am weak then am I strong. Take grace into your heart, rely on Gods grace for strength, with grace in your heart your feeble knees will be strengthened. (See HART OF GREASE.)