What does it mean to be zealous? To be consumed, eaten up with an idea or truth that has gripped your heart. You stake your whole life upon it and are willing to do whatever it takes to further the cause and see it's purpose and goals achieved. There is a definite sense of urgency. Galatians 4:18 says it is good to be always zealously affected and Titus 2:14 says that believers are meant to be zealous of good works.
One book that was spiritually beneficial to me during my college years was Practical Religion by J.C. Ryle. I came across this book on my Dad's bookshelf and took it with me to college. Ryle has many excellent, relevant chapters about living the Christian life and it was such a treasure to discover, one such chapter was on zeal. I was recently going over old files on the laptop my sister and I shared in college, when I came across these quotes I had typed up a while ago. I was freshly encouraged and provoked to be a truly zealous Christian, because our time on earth is short, we have to make our lives count for 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, through-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 is has health, or whether he has sickness, -whether he is rich, or whether he is poor, -whether he pleases man, or whether he gives offense, -whether he is thought wise, or whether he is thought foolish, -whether he gets blame, or whether gets praise, -whether he gets 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 nothing 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 an one will always find a sphere for his zeal. If he cannot preach, and work, and give money, he will cry, and sigh, and pray. ~Practical Religion, pg.184-185
Think of the shortness of time. You will soon be gone. You will have no opportunity for works of mercy in another world. In heaven there will be no ignorant people to instruct, and no unconverted to reclaim. Whatever you do must be done now. Oh, when are you going to begin? Awake! Be zealous, and repent. Think of the devil, and his zeal to do harm. It was a solemn saying of old Bernard when he said that "satan would rise up in judgment against some people at the last day, because he had shown more zeal to ruin souls than they had to save them." Awake! Be zealous, and repent. ~Practical Religion, pg.208