Zeal: Necessary, Dangerous

Lukewarmness in religion is abominable, and zeal an excellent grace; yet above all other Christian virtues, it needs to be strictly watched and searched; for 'tis that with which corruption, and particularly pride and human passion, is exceeding apt to mix unobserved. 

--Jonathan Edwards, The Distinguishing Marks of a Work of the Spirit of God, in Works, Yale edition, 4:243