Is smoking a sin?
That depends. If one sincerely believes something to be either a legitimate detriment to his health, or finances, usefulness to others, or the honor of God whom he represents, then yes, it is.
The same is true of coffee, red meat, and anything else existing in material form. Suppose a man thinks an occasional steak maintains his health. He is then free to eat and be full, and thank the Lord with a good conscience for his meal. Contrarily, if that man eats five steaks a week at $25 a sitting, while being convinced this diet is harming both his body and his willingness to share generously with others - and yet goes on in the habit - then it is a sinful practice which must be stopped.
One may be surprised to learn that tobacco was considered by some in the 17th century to offer valuable medicinal benefits. Pipe smoking was recommended as an effective treatment for migraines, as noted in Daniel Defoe's classic, 'Robinson Crusoe.' Under these impressions such smokers were by no means guilty of sin. This is because believers have freedom in Christ to partake in every creature, provided doing so is not an impediment to Christian love. It is the mark of a heretic to forbid others to eat meats or to marry. [1 Tim. 4:3-5] We ought not to forbid smoking, per se, but to warn against any knowledgeable abuse of the physical body, which is unloving towards those we might have ministered to with our full strength.
Confronting the issue of chemical addictions and deeply set habits is overwhelming for many. They have tried so many methods without success, even praying and fasting at length, and resort to frightening themselves and making rash oaths. Fear of God's anger or the disapproval of others may hold one off for a time. But fear is a weak motive and often proves inferior to strong cravings.
The real problem lies in having insufficient motives for quitting negative habits. Until one's motive to stop exceeds the motive to continue, there will be no permanent resolve. What motive is capable of providing enough strength to abstain entirely from something one enjoys? Only a superior satisfaction in the benefits of abstaining from it.
In the case of harmful habits, one needs true conviction that his abstinence is for the best of God and others, rather than for himself. His unwillingness stems from a depravity of sincere brotherly love, which ought to be mourned for, but not despaired over. God is powerful to change hearts: on His resume is the creation of new and clean ones. Those who have repeatedly fought and fallen before temptation may place hope in the One who "renews a steadfast spirit within." [Psm 51:10] It is more love that is needed, and such love is a fruit of the Spirit [Gal 5]. He has power to produce that virtue in everyone who faithfully attends the means of grace.
In conclusion, habitual sin is conquered by superior desires to love God and others. This love cannot be urged from one's natural self, therefore must be supernaturally wrought by the Spirit in believers. This work was purchased by Christ, and is received by faith. As a Christian rests in Christ's righteousness to appropriate his sanctification, he receives a transformation of heart through the means of prayer and the Word.
I exhort every addict to find his superior satisfaction in loving God and men, and to draw nerve-calming peace from the promises of Christ, the [i]Stimulus Ultimum[/i].
My two cents. God bless you.