When Jesus addressed the Christians of this city in the book of Revelation, he used an illustration they would understand: "I know your deeds, that you are neither cold nor hot. I wish you were either one or the other! So, because you are lukewarm — neither hot nor cold — I am about to spit you out of my mouth" (Revelation 3:15-16). The Greek word translated "spit" is more accurately translated "vomit." It's an unpleasant image for an unpleasant condition: Christians who have lost their passion, who have compromised their loyalty to Christ.
Unfortunately, the world has seen too many Christians just like this. Mahatma Gandhi said, "I like your Christ, I do not like your Christians. Your Christians are so unlike your Christ." The agnostic Sheldon Vanuaken wrote,
The best argument for Christianity is Christians: their joy, their certainty, their completeness. But the strongest argument against Christianity is also Christians--when they are sombre and joyless, when they are self-righteous and smug in complacent consecration, when they are narrow and repressive, then Christianity dies a thousand deaths.
If you're not a Christian and that's the only Christianity you've seen, then I'd urge you to look again. There are Christians who have overcome this perpetual state of apathy. They know that Christianity isn't just about showing up at church at certain times. It's a way of life. As Vanuaken said, such Christians are the best argument for Christianity that exists.
If you are a Christian, but find that lukewarm describes your faith, let me encourage you to go back to the basics. Focus on Bible study and prayer. Find a church and make yourself an active part of it. Reconnect with God each week by participating in the Lord's Supper.