I love the Lord dearly and have a very close walk with Him... but I smoke. A part of me knows that it is not right and I am careful to not smoke in front of people that I may cause to stumble because of my bad habit; but I enjoy it. And that to me is a problem. Is smoking a sin?
You’ve answered your own question! Anything that causes your redeemed conscience to condemn you (1 Tim 1:19) or that could cause a fellow Christian to stumble is viewed as sin in the eyes of God (1 Cor 8:12).
It is also worth thinking about the fact that your body is the very Temple of God and that the Spirit of God dwells in you (1 Cor 3:16). Verse 17 goes on to say that anyone who defiles (does damage to) the Temple is in serious trouble with God “for the Temple of God is holy, which temple you are.” Ask yourself the question: Does smoking damage my body in any way; is it helpful or hurtful? We have been given a very serious responsibility by God to care for this earthly body which was purchased by Him and which shall one day be raised as a glorious body. Smoking is a tenacious habit, but the Lord can set you free in a moment of time. Just ask Him for “he whom the Son sets free, is free indeed” John 8:36.