It should be added, however, in order to a correct estimation of this matter, that strength or energy of character cannot be well explained without a reference to the will. And in accordance with this remark, the common idea of an energetic man is, that he is a person of a strong or energetic will. There are diversities in the constitution of the will, it is true; and as a result of this, there are diversities in personal energy; just as there are diversities in other elements and traits of character. Some men, in consequence of possessing original strength of will, are naturally more decided, more energetic than others. But other things being equal, in other words, on the supposition of there being no such constitutional differences between them as have been indicated, that person, as compared with others, will be the most energetic, who has the strongest faith. The believing man will be the strongest man.
— edited from The Life of Faith (1852) Part 1, Chapter 2.