I am sure many Latter-day Saints who like to debate Mormonism on the Internet think that they are smart enough to take care of themselves, and don’t need any advice from me—and they are probably right. This is intended for those who may find it useful.
An important lesson to learn from debating Mormonism on the Internet is knowing when not to reply to someone. It often happens that the best reply to give to someone is not to reply to them at all! In those situations, any other answer given will be the wrong answer, and will put one at a disadvantage. It is difficult to lay down hard and fast rules when exactly to do that. It is like “teaching by the Spirit” (D&C 42:12). If you have the Spirit, you will know.
There are some people who as a general rule one should not reply to at all, except on rare occasions, and under the “constraint of the Spirit”:
D&C 63:64 “Remember that that which cometh from above is sacred, and must be spoken with care, and by constraint of the Spirit; and in this there is no condemnation, and ye receive the Spirit through prayer; wherefore, without this there remaineth condemnation.”
There are people out there who are in denial of the Holy Ghost; and it is actually a sin to engage them in debate:
1 John 5:16 “If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it.”
2 John 1:10–11 “If there come any unto you, and bring not this doctrine, receive him not into your house, neither bid him God speed: For he that biddeth him God speed is partaker of his evil deeds.”
So if we are not supposed to pray for them, and even say hello and good bye to them, then it stands to reason that we should not debate with them either. There are such folks around; and they tend to congregate on Christian message boards on the Internet. We recognize them by the Holy Ghost. (Apostates from the Church sometimes fall into this category.) This does not mean that one should never reply to them. On rare occasions it might be necessary to do so. But there is a difference between giving them an occasional appropriate response, and engaging them in lengthy and protracted debate. Jesus gave appropriate responses to the devil when He was being tempted by him (Matthew 4:1–11); but He did not sit down and argue with him!
We also need to decide whether we want to inform people or argue with them. There is a difference between somebody who has a genuine disagreement with us and is willing to engage in a genuine discussion about those differences, and somebody who has no interest in the answers you give and just wants to argue. Again, we recognize them by the Holy Spirit.