Is it a sin to smoke or eat junk food?
I think it is a sin to smoke, in general, because of the scientific studies that have come out today. Maybe once upon a time, when we didn't know any better, it wouldn't have been a sin. But now that we know how destructive it is and we see its deadly effects printed right on the side of the carton, it seems really strange to me that Christians would embrace it.
Habitually smoking cigarettes seems to say, "Life doesn't matter as much as my pleasures do." And the fact that it is highly addictive should also encourage Christians to keep their distance.
Are there other things that, because of their similar nature, would classify as sin? I think so.
Paul said, "I will not be enslaved by anything" (1 Corinthians 6:12). That means caffeine, food, nicotine, or crack. I'm not going to be enslaved by anything. I want my mind and my body to be alert, supple, responsive, and ready to follow and obey Jesus.
We have a church covenant at Bethlehem. There is a line in it that, when I came, demanded that no member be allowed to drink alcohol. I thought that such a demand was unbiblical and I forced the issue very soon after I came. It was a risky thing to do for a young pastor.
And I was criticized for it. But my response was that we were not making the standard lower. Rather, we were making it higher. Instead of saying, "We engage to abstain from the use and sale of alcohol," the covenant now says, "We engage to abstain from all drugs, food, drink, and practices which bring unwarranted harm to the body or jeopardize our own or another's faith."
That is a very radical statement, because it says that if eating sugar, drinking caffeine, or having a second helping jeopardizes your own faith or another's faith, don't do it. That's based on 1 Corinthians 8 & 10 where we're commanded to not hurt the weaker brother. Nor are we to undermine our own body, which is the dwelling place of the Holy Spirit.
So don't pick out a few individual named sins, but let your definition of sin be sweeping and pervasive so that it touches everything. And let your definition of holiness be the same. How you drink Coke, eat pizza, or exercise can all be sin issues, or not.
If smoking is sinful then should we exercise church discipline on all smokers?
You'd have to see if your elders were in agreement on that.
If I knew someone who was a member at my church and who smoked, I would talk to them about it. I would ask why they were doing it. And if they gave me a blasé response, then, yes, maybe it would be.
But if they said, "I'm working on it. I'm struggling with it," then I would go a long way with a struggler.
Not all sinning results in church discipline. It is usually those sins which are public, flagrant, and continual.
If you're not even trying to quit but are giving up to self-destructive, suicidal behavior, that's wrong. Now whether it is wrong enough to exclude you from church membership, I'm not yet sure.