The zealot according to knowledge, motivated by the love of God and his neighbor, does all things with love and self-effacement; he does nothing that might bring sorrow to his neighbor; such a zealot is enlightened by knowledge and nothing prompts him to deviate from what is morally right.
The Christian who is inspired by zeal not according to knowledge is a ruinous man who turns the Gospel of Grace and love upside down. His zeal is a seductive fire, a consuming fire. Destruction comes forth from him and desolation follows in his wake. He beseeches God to send down fire from Heaven and to devour all of those who do not embrace his principles and convictions. He is characterized by hatred for those of other religions and confessions, envy and persistent anger, violent resistance to the true spirit of Divine Law, an unreasonable obstinacy in defending his own views, a passionate zeal for prevailing in all things, the love of glory, quarrels, contention, and a love of turmoil.
~St. Nektarios of Aegina
(hat tip: Holy Fathers Page on FB)