Romans 14:21It is good neither to eat flesh, nor to drink wine, nor any thing whereby thy brother stumbleth, or is offended, or is made weak.
See All... says, "It is good neither to eat meat nor drink wine nor so anything by which your brother stumbles or is offended or is made weak", so if one of my Christian buddies is doing something that causes me to stumble then they should stop? And if they don't, what will be their consequences?

The larger context of this verse reads as follows:

Romans 14:20-23 [20] For meat destroy not the work of God. All things indeed are pure; but it is evil for that man who eateth with offence. [21] It is good neither to eat flesh, nor to drink wine, nor any thing whereby thy brother stumbleth, or is offended, or is made weak. [22] Hast thou faith? have it to thyself before God. Happy is he that condemneth not himself in that thing which he alloweth. [23] And he that doubteth is damned if he eat, because he eateth not of faith: for whatsoever is not of faith is sin.
See All... For meat destroy not the work of God. All things indeed are pure; but it is evil for that man who eateth with offence. It is good neither to eat flesh, nor to drink wine, nor any thing whereby thy brother stumbleth, or is offended, or is made weak. Hast thou faith? have it to thyself before God. Happy is he that condemneth not himself in that thing which he alloweth. And he that doubteth is damned if he eat, because he eateth not of faith: for whatsoever is not of faith is sin.

Here is an important parallel passage:

1 Corinthians 8:9-13 [9] But take heed lest by any means this liberty of your's become a stumblingblock to them that are weak. [10] For if any man see thee which hast knowledge sit at meat in the idol's temple, shall not the conscience of him which is weak be emboldened to eat those things which are offered to idols; [11] And through thy knowledge shall the weak brother perish, for whom Christ died? [12] But when ye sin so against the brethren, and wound their weak conscience, ye sin against Christ. [13] Wherefore, if meat make my brother to offend, I will eat no flesh while the world standeth, lest I make my brother to offend.
See All... But take heed lest by any means this liberty of yours become a stumblingblock to them that are weak. For if any man see thee which hast knowledge sit at meat in the idol's temple, shall not the conscience of him which is weak be emboldened to eat those things which are offered to idols;  11 And through thy knowledge shall the weak brother perish, for whom Christ died?  12 But when ye sin so against the brethren, and wound their weak conscience, ye sin against Christ.  13 Wherefore, if meat make my brother to offend, I will eat no flesh while the world standeth, lest I make my brother to offend.

In 1 Corinthians 8:12But when ye sin so against the brethren, and wound their weak conscience, ye sin against Christ.
See All..., unnecessarily offending weak brothers by our blatant acts of liberty is called sin - "when we sin so against the brethren." Romans 14:22Hast thou faith? have it to thyself before God. Happy is he that condemneth not himself in that thing which he alloweth.
See All... states that we can condemn ourselves in those things which we allow. Romans 14:23And he that doubteth is damned if he eat, because he eateth not of faith: for whatsoever is not of faith is sin.
See All... continues, "And he that doubteth is damned if he eat, because he eateth not of faith: for whatsoever is not of faith is sin." This is strong language. However, there is no reason to think that the offending brother is eternally damned in hell. He is a brother-a saved man-and the sin is that of not caring properly for a weaker brother. This is not a cause for losing salvation in anybody's theology.

The damnation is a declaration of strong disapproval: he "condemneth...himself" (Romans 14:22Hast thou faith? have it to thyself before God. Happy is he that condemneth not himself in that thing which he alloweth.
See All...). It specifically means to declare the sentence of judgment against someone. In several cases in the Bible, it clearly refers to eternal damnation (Mark 16:16He that believeth and is baptized shall be saved; but he that believeth not shall be damned.
See All...; 2 Thessalonians 2:12That they all might be damned who believed not the truth, but had pleasure in unrighteousness.
See All...). However, we can tell that this is not always the meaning of the word because of the need for occasional descriptive words or phrases to define the damnation as eternal: as in "the damnation of hell" (Matthew 23:33Ye serpents, ye generation of vipers, how can ye escape the damnation of hell?
See All...) and "eternal damnation" (Mark 3:29But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation.
See All...).

Therefore, there are times when the Bible concept of damnation refers to a sentence of judgment against someone without the experience of hell. A couple of examples follow:

Romans 13:2Whosoever therefore resisteth the power, resisteth the ordinance of God: and they that resist shall receive to themselves damnation.
See All... Whosoever therefore resisteth the power, resisteth the ordinance of God: and they that resist shall receive to themselves damnation.

1 Corinthians 11:29For he that eateth and drinketh unworthily, eateth and drinketh damnation to himself, not discerning the Lord's body.
See All... For he that eateth and drinketh unworthily, eateth and drinketh damnation to himself, not discerning the Lord's body.

As concerning the warning in 1 Corinthians 11:29For he that eateth and drinketh unworthily, eateth and drinketh damnation to himself, not discerning the Lord's body.
See All..., the next verse explains some of the results of this damnation: "For this cause many are weak and sickly among you, and many sleep" (1 Corinthians 11:30For this cause many are weak and sickly among you, and many sleep.
See All...). Because of their sin, some were sick and others had gone to an early grave. However, they were still saved as indicated by the use of the word "sleep" for death. This is a biblical reference to the temporary nature of the death of a believer. It is not used of the lost.

The sin referred to in Romans 14:22-23 [22] Hast thou faith? have it to thyself before God. Happy is he that condemneth not himself in that thing which he alloweth. [23] And he that doubteth is damned if he eat, because he eateth not of faith: for whatsoever is not of faith is sin.
See All... is not eating "of faith." This must refer to the act of eating without a clear knowledge of faith that this is the very thing that the Lord wanted the person to do. Christians are not to play with their liberty, but are to temper it with their responsibility toward others. It was in regard to such questions that Paul said, "For none of us liveth to himself, and no man dieth to himself" (Romans 14:7For none of us liveth to himself, and no man dieth to himself.
See All...). We are responsible for how our life effects the lives of others.

If we take this responsibility lightly and freely participate in activities that hurt the faith of others, we condemn ourselves by our own actions. That is, we declare a sentence of guilt against ourselves. We have used our "liberty for a cloke of maliciousness" (1 Peter 2:16As free, and not using your liberty for a cloke of maliciousness, but as the servants of God.
See All...) and have hurt others for the opportunity to live as we please and not to please the Lord. The result of this "damnation" is not defined in the text. It may not be any more than is suffered for any sin. Paul emphasizes it because many saw it as no sin at all. They felt they could freely do anything God did not specifically prohibit. But Paul is trying to shake them and make them realize that caring for the brethren is a grave responsibility for the believer. We, after all, are our brother's keeper.