Zeal in religion is a burning desire to please God, to do His will, and to advance His glory in the world in every possible way. It is a desire, which no man feels by nature--which the Spirit puts in the heart of every believer when he is converted--but which some believers feel so much more strongly than others that they alone deserve to be called "zealous" men.
This desire is so strong, when it really reigns in a person, that it impels them to make any sacrifice-to go through any trouble-to deny themselves anything--to suffer, to work, to labour, to toil, to spend themselves and be spent, and even to die--if only they can please God and honour Christ.
A zealous man in religion is pre-eminently a man of one thing. It is not enough to say that he is earnest, hearty, uncompromising, thorough-going, whole-hearted, fervent in spirit. He only sees one thing, he cares for one thing, he lives for one thing, he is swallowed up in one thing; and that one thing is to please God. Whether he lives, or whether he dies—whether he has health, or whether he has sickness—whether he is rich, or whether he is poor—whether he pleases man, or whether he gives offence—whether he is thought wise, or whether he is thought foolish—whether he gets blame, or whether he gets praise—whether he get honour, or whether he gets shame—for all this the zealous man cares nothing at all. He burns for one thing; and that one thing is to please God, and to advance God’s glory. If he is consumed in the very burning, he cares not for it--he is content. He feels that, like a lamp, he is made to burn; and if consumed in burning, he has but done the work for which God appointed him. Such a one will always find a sphere for his zeal. If he cannot preach, work, and give money, he will cry, and sigh, and pray. Yes: if they are extremely poor, on a perpetual bed of sickness, they will make the activity of sin around him slow to a standstill, by continually interceding against it. If he cannot fight in the valley with Joshua, he will do the work of Moses, Aaron, and Hur, on the hill (Exodus 17:9-13). If he is cut off from working himself, he will give the Lord no rest till help is raised up from another quarter, and the work is done. This is what I mean when I speak of ‘zeal’ in religion.