The Religious Affections by Jonathan Edwards

God contrived and constituted things, in his dispensations towards his own people, that when their love decays, and the exercises of it fail, or become weak, fear should arise; for then they need it to restrain them from sin, and to excite them to care for the good of their souls, and so to stir them up to watchfulness and diligence in religion: but God hath so ordered, that when love rises, and is in vigorous exercise, then fear should vanish, and be driven away; for then they need it not, having a higher and more excellent principle in exercise, to restrain them from sin, and stir them up to their duty. There are no other principles, which human nature is under the influence of, that will ever make men conscientious, but one of these two, fear or love; and therefore, if one of these should not prevail as the other decays, God's people, when fallen into dead and carnal frames, when love is asleep, would be lamentably exposed indeed: and therefore God has wisely ordained, that these two opposite principles of love and fear should rise and fall, like the two opposite scales of a balance; when one rises the other sinks.

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