The Bible actually tells us what a "lukewarm" Christian is; it's someone who's neither hot nor cold about their faith (see Revelation 3:14-16).
In other words, their faith doesn't mean very much to them. They aren't particularly enthusiastic about it, nor are they hostile toward it. Instead, they drift along, perhaps going to church when it's convenient or giving some money to God's work -- but never sacrificing to help others or going out of their way to share Christ with someone. Instead of loving God with all their heart, they are indifferent toward Him, and think about Him mainly when they get in trouble.
The Bible says a startling thing about a lukewarm faith: It tells us that God detests it. He even warns, "So, because you are lukewarm -- neither hot nor cold -- I am about to spit you out of my mouth" (Revelation 3:16). Does that sound harsh? It may at first -- but remember: Jesus Christ left the glory of heaven and willingly shed His blood on the cross for us. He loves us that much! How can we remain indifferent to Him?
If this describes you, don't despair. God wants to forgive you and draw you closer to Himself -- and He will, as you repent of your sin and commit yourself without reserve to Christ. Begin a new journey with Christ today. Then get in a church where Christ is preached and lived, so you can continue to grow in your faith. You'll never regret it.