(1 Corinthians 6:12-13) "All things are lawful for me", but not all things are profitable. "All things are lawful for me", but I will not be mastered by anything.
The phrase "All things are lawful for me" was a quote used frequently by the Corinthians in order to keep themselves comfortable in their antinomianism. People were indulging in the flesh without any control over themselves. Paul countered their quote with a more biblical and God-honoring minded alternative. The verse is specifically referring to food, and being mastered by food would of course be gluttony. The important thing to be derived from this text is the principle of not being mastered by anything. If you are being mastered by something, then there is work to do, a battle to wage, and sanctification to pursue. This would involve anything that enslaves anybody, including smoking/drinking, playing video games, overeating, no control over spending, various addictions, etc. As people of God we are slaves to one, that is Jesus Christ. Everything else must be fought against.
(Romans 6:16) Do you not know that when you present yourselves to someone as slaves for obedience, you are slaves of the one whom you obey, either of sin resulting in death, or of obedience resulting in righteousness?
If you yield your body to the addictive act of smoking, you are presenting yourself as a slave to it. Yet we must not be "mastered by anything," save Christ.
Just because a Christian smokes does not necessarily mean they're not a Christian. Whether or not a person smokes is not the dividing factor determining salvation or not. Some Christians smoke and are ignorant of it...but if they are God's, then He will sooner or later bring them to conviction. Some Christians smoke and are struggling with it; they have their ups and downs, but the important thing is there's a fight. Now people who claim to be Christian and are aware of these Scriptures and how we must not yield ourselves as slaves to anything, and they have no concern for striving in greater sanctification...these are the ones that should examine themselves to see whether or not they are in the faith.
If you want to see a short 3 or so minute question & answer type explanation on this subject, please visit the following page. It is Pastor John Piper giving his input on the matter:
http://www.desiringgod.org/Resource...smokes_does_that_mean_theyre_not_a_Christian/