If it is legal, and the person using it does so in a moderate, responsible way, is the use of marijuana immoral?
It seems to be on the way to legality. A number of States have passed the medical marijuana. It is certainly a safer drug than alcohol in many ways.
Here are some basic principles that may applied to ethical decision making:
1 Cor 6:12-15
“All things are lawful for me,” but not all things are beneficial. “All things are lawful for me,” but I will not be dominated by anything. 13 “Food is meant for the stomach and the stomach for food,” and God will destroy both one and the other. The body is meant not for fornication but for the Lord, and the Lord for the body. 14 And God raised the Lord and will also raise us by his power. 15 Do you not know that your bodies are members of Christ?
Some questions to ask with regard to morality might be:
Is it beneficial to me?
Does it get me in it’s power? Will I become dominated by it?
Is the Body of Christ benefitted?
1 Cor 6:19-20
19 Or do you not know that your body is a temple of the Holy Spirit within you, which you have from God, and that you are not your own? 20 For you were bought with a price; therefore glorify God in your body.
Does it glorify God?
This next principle (expediency) Paul describes with regard to food offered to idols. However, the principle can apply to other liberties as well:
1 Cor 8:8-13
We are no worse off if we do not eat, and no better off if we do. 9 But take care that this liberty of yours does not somehow become a stumbling block to the weak. 10 For if others see you, who possess knowledge, eating in the temple of an idol, might they not, since their conscience is weak, be encouraged to the point of eating food sacrificed to idols? 11 So by your knowledge those weak believers for whom Christ died are destroyed. 12 But when you thus sin against members of your family, and wound their conscience when it is weak, you sin against Christ. 13 Therefore, if food is a cause of their falling, I will never eat meat, so that I may not cause one of them to fall.
Does it cause a stumbling block to others? If another witnesses your behavior, will it be a cause for scandal? Might your liberty result in leading a brother astray who cannot afford such liberty?
This last passage has to do with evangelism:
1 Cor 10:23-11:1
“All things are lawful,” but not all things are beneficial. “All things are lawful,” but not all things build up. 24 Do not seek your own advantage, but that of the other. 25 Eat whatever is sold in the meat market without raising any question on the ground of conscience, 26 for “the earth and its fullness are the Lord’s.” 27 If an unbeliever invites you to a meal and you are disposed to go, eat whatever is set before you without raising any question on the ground of conscience. 28 But if someone says to you, “This has been offered in sacrifice,” then do not eat it, out of consideration for the one who informed you, and for the sake of conscience — 29 I mean the other’s conscience, not your own. For why should my liberty be subject to the judgment of someone else’s conscience? 30 If I partake with thankfulness, why should I be denounced because of that for which I give thanks?
31 So, whether you eat or drink, or whatever you do, do everything for the glory of God. 32 Give no offense to Jews or to Greeks or to the church of God, 33 just as I try to please everyone in everything I do, not seeking my own advantage, but that of many, so that they may be saved.
Will engaging in this behavior benefit others? Does it give glory to God? Does it seek the advantage of others as well as oneself?