In scripture, the Christian Bible and the Islamic Qur'an give some very similar instructions regarding debating style. Believers must continue to debate patiently, courteously, kindly and with respect whenever they see or hear their own doctrine being absent or ignored. Believers shouldn't give up on trying to convince someone, saying things like "let's leave this until later", or "I don't want to talk about it" when asked difficult questions. Also, believers should not make assertions, saying "this is so!" without providing any proper information before the exclamation marks.
The first verses we look at from the Bible says that Christians must give reasonable answers to those who ask, whereas some other verses say that preaching must be active and continuous - presumably whether or not people have asked for it.
“Always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have. But do this with gentleness and respect, keeping a clear conscience, so that those who speak maliciously against your good behaviour in Christ may be ashamed of their slander.”
1 Peter 3:15-16 (NIV)
“Preach the Word; be prepared in season and out of season; correct, rebuke and encourage - with great patience and careful instruction.”
2 Timothy 4:2 (NIV)
“Let your utterance be always with graviousness, seasoned with salt, so as to know how you ought to give an answer to each one.”
Titus 3:2 is of a similar vein, saying to avoid slandering people, and to be peaceable and considerate. And 1 Timothy 3:1-11 repeats these instructions, in particular applying them as the necessary style of senior Christians and their wives (see 1 Tim 3:7 in particular - "good report of those without" means he must speak well of non-Christians) . And Proverbs 15:1 says that "a soft answer turns away wrath, but a harsh word stirs up anger" - with the implication that the former route is better than the latter.
Not all verses are clear. Ephesians 4:24-25 says "therefore each of you must put off falsehood and speak truthfully to your neighbor, for we are all members of one body" [NIV] and it is hard to see if "neighbour" therefore only means they "of one body", ie. the Christian community. So it might mean "don't lie to Christians", or, if it is counting all humans as "one body", then, it means "don't lie". In reality, it doesn't really matter, because the irony is that Ephesians is a forgery written in the name of Paul1, and this warning was likely put there to make it seem more legitimate.
But this niceness isn't quite the whole story, as some verses in the Bible state that you shouldn't take the gentle, polite and respectful route. 2 John says that if you don't have the right beliefs about the relationship between Jesus-as-god and Jesus-as-man then you are godless (2 John 1:7-9), and Christians can't greet you politely nor welcome you in to church or home (2 John 1:10-11). Just to greet people with wrong beliefs, says 2 John, is to be in league with evil! This has no doubt helped encourage the intolerant and fundamentalist streams in Christian history.
"God does not love harsh words [in public], except when uttered by a man who is truly wronged" (Qur'an 4:148). The words "in public" appear in the classical Mohsin translation of the Qur'an, but not in the Dawood translation.
Muslims should "invite" (da'wah) others to Islam "with wisdom and beautiful preaching and dispute with them in the better manner", or according to the Dawood translation, "with wisdom and kindly exhortation. Reason with them in the most courteous manner" (Qur'an 16:25 - I think I've messed up the reference for this, 16:25 in the Qur'ans I have now don't match these notes!).
Such debating must occur when non-Islamic beliefs are encountered:
"When you hear god's revelation being denied or ridiculed you must not sit and listen to them until they engage in other talk, or else you yourselves become like them" (Qur'an 4:140).
The community of skeptics and freethinkers is often scattered with people who are so critical in their thinking that they forget basic human manners. From time to time, this is acknowledged as a problem by the community at large, and some have called for a self-conscious effort for evidence-driven debaters to find ways of being nicer with their refutations. One such time was at an annual conference of skeptics called The Amazing Meeting in 2010:
“TAM8 acknowledged the importance of outreach and grassroots activism, but it also recognized the need to improve the public image of skepticism. The message was that we can increase our effectiveness and increase our numbers by simply being... nicer.
We all know the stereotypes of skeptics. There is the boring, bearded, bespectacled old man. There is the cynical and cantankerous curmudgeon. There is the self-righteous, smug, superior know-it-all. Rather than worrying about gender or age, we should worry about approaches and attitudes. [...]
Phil Plait also tackled these themes. [He] voiced his concerns over the public perception of skeptics as antisocial, egotistical, and abrasive. [...] Plait's plea was that we should avoid undue attacks and insults because we're most persuasive when we're respectful and rational.
Of course, there's a time to be confrontational, but there's also a time to be considerate. Such animosity is often directed towards believers.”