Henry Scougal (1630-1657), professor of divinity at Aberdeen University until dying of tuberculosis at age 28 (oddly similar to Brainerd), wrote a letter to a friend which was later turned into a small book called The Life of God in the Soul of Man. It was instrumental in the conversion of George Whitefield. Scougal writes these words, relevant for Christian motivation:
"[R]eligion may be designed by the name of life; because it is an inward, free and self-moving principle; and those who have made progress in it, are not acted only by external motives, driven merely by threatenings, nor bribed by promises, nor constrained by laws; but are powerfully inclined to that which is good, and delight in the performance of it."
"The love which a pious man bears to God and goodness, is not so much by virtue of a command enjoining him to do so, as by a new nature instructing and prompting him to it; nor doth he pay his devotions as an unavoidable tribute, only to appease the Divine justice, or quiet his clamorous conscience; but those religious exercises are the proper emanations of the Divine life, the natural employments of the new-born soul. He prays, and gives thanks, and repents, not only because these things are commanded, but rather because he is sensible of his wants, and of the Divine goodness, and of the folly and misery of a sinful life; his charity is not forced, nor his alms extorted from him, his love makes him willing to give; and though there were no outward obligation, his 'heart would devise liberal things'; injustice or intemperance, and all other vices, are as contrary to his temper and constitution, as the basest actions are to the most generous spirit, and impudence and scurrility to those who are naturally modest."
"The love of God is a delightful and affectionate sense of the Divine perfections, which makes the soul resign and sacrifice itself wholly unto him, desiring above all things to please him, and delighting in nothing so much as in fellowship and communion with him, and being ready to do or suffer any thing for his sake, or at his pleasure. Though this affection may have its first rise from the favours and mercies of God toward ourselves, yet doth it, in its growth and progress, transcend such particular considerations, and ground itself on his infinite goodness, manifested in all the works of creation and providence."
"When we have said all that we can, the secret mysteries of a new nature and divine life can never be sufficiently expressed; language and words can not reach them; nor can they be truly understood but by those souls that are enkindled within, and awakened unto the sense and relish of spiritual things."
--The Life of God in the Soul of Man (Fearn, Scotland: Christian Focus, 2001), 43-55.