That religion which God requires, and will accept, does not consist in weak, dull, and lifeless wishes, raising us but a little above a state of indifference: God, in his word, greatly insists upon it, that we be good in earnest, “fervent in spirit,” and our hearts vigorously engaged in religion. . .
If we be not in good earnest in religion, and our wills and inclinations be not strongly exercised, we are nothing.
The things of religion are so great, that there can be no suitableness in the exercises of our hearts, to their nature and importance, unless they be lively and powerful. In nothing is vigor in the actings of our inclinations so requisite, as in religion; and in nothing is lukewarmness so odious. True religion is evermore a powerful thing; and the power of it appears, in the first place in the inward exercises of it in the heart. . . .
And such, when they receive the Spirit of God, in his sanctifying and saving influences, are said to be “baptized with the Holy Ghost, and with fire;” by reason of the power and fervor of those exercises the Spirit of God excites in their hearts, whereby their hearts, when grace is in exercise, may be said to “burn within them;” as is said of the disciples (Luke 24:32).
Jonathan Edwards, Religious Affections, Part 1, Ch 2.