Is it obligatory to continue debating with a non-Muslim who insults, curses, and sends specious arguments, and to prepare rebuttals to them, or should one turn away from him? And is it correct to endure his harm, or is it permissible to defend oneself and retaliate in kind?

Debating those who oppose the truth should only be undertaken by those who are qualified. Regarding responding to Christians, it is advisable to direct the inquirer's colleague to books or websites specialized in refuting their misconceptions, such as the book "Izhar al-Haqq" by Sheikh Rahmatullah al-Hindi or its abridged version. As for the issue of insult, it is not befitting for a caller to Islam. It should be clarified that a Muslim does not insult because his religion encourages purity of speech and arguing in the best manner, following the example of the Prophet (peace be upon him) who was never obscene, but rather pardoned and forgave, even his enemies.