Romans 12:11 (New International Reader's Version)
11 Never let the fire in your heart go out. Keep it alive. Serve the Lord.
Romans 12:11 (New International Version, ©2011)
11 Never be lacking in zeal, but keep your spiritual fervor, serving the Lord.
Spiritual fire, a filling of God's Holy Spirit, the reception of Jesus promised "Comforter" is the mark of a follower of Jesus Christ. When you are "saved", surrender your life and accept the sacrificial death of Jesus on the cross for your sins, and accept Him as Lord over your life, as the risen, living, Son of God, you are filled with a constant companion. He helps guide you through this Christian life, convicting you of wrong and lifting you up in times of righteousness and joy.
I believe that once you experience a true coversion, you can never lose it, but Paul encourages us to never lose our zeal for God; never get bored with serving Him. After a few years in our Christian lives, we may tire of waiting for our coming Savior, but keep in mind that it is all working out ofr His purposes. Be about His business, and He will be about yours!
Photo "Worship" by Everywhere, 12-30-2005, from stock.xchng