Praise be to Allah.
What you mention about the activities of some missionaries and their patience in travelling and making sacrifices is something that is well known. If this is their state even though they are promoting falsehood and striving for the sake of falsehood, then how about the Muslim whom Allaah has blessed with guidance and taught him the path of truth? Undoubtedly he, of all people, should be striving and making the effort, sacrificing that which is most dear to him, in order to spread the religion of Islam and teach and guide people, whether they are Christians or others. The Prophet (peace and blessings of Allaah be upon him) said: “Advance cautiously, until you reach their open space, then invite them to Islam, and tell them of their duties before Allaah. By Allaah, if Allaah were to guide one man through you, that would be better for you than having red camels.” Narrated by al-Bukhaari (3009) and Muslim (2406).
The path of da’wah (calling others to Islam) is the path of the Prophets and Messengers, because they were the most compassionate of people towards people, and the keenest of people to guide people. But it is a path that needs provision, namely knowledge, patience and certainty of faith. You have done well to ask about the way to specialize in debating with the Christians, because some people indulge in this field without preparation, and that may lead to his putting his opponent off or making him cling more steadfastly to his falsehood, or it may lead to him becoming weak himself and being filled with doubts.
Hence the one who wants to strive for the sake of Allaah and call these people to Islam and debate with them should pay attention to a number of matters:
1 – Studying sound ‘aqeedah from reliable sources, directly from scholars if possible.
2 – Acquiring shar’i knowledge which will correct his worship and interactions with others, because learning this is an individual obligation, and it takes precedence over the duty of calling others.
3 – Paying attention to Islamic sources which deal specifically with calling Christians to Islam and discussing their situation and their specious arguments. There are many such sources, among the most important of which are: al-Jawaab al-Saheeh li man baddala Deen al-Maseeh by Shaykh al-Islam Ibn Taymiyah; Hidaayat al-Hayaara fi Ajwabah al-Yahood wa’l-Nasaara by Ibn al-Qayyim; Izhaar al-Haq by Shaykh Rahmat-Allaah al-Hindi (available in English); Manaazirah bayna al-Islam wa’l-Nasraaniyyah by Shaykh Muhammad Jameel Ghaazi; al-A’qaa’id al-Wathaniyyah fi’l-Diyaanah al-Nasraaniyyah by Muhammad Taahir al-Tanneer; Muhaadiraat fi’l-Nasraaniyyah by Shaykh Abu Zahrah; al-Mawsoo’ah al-Muyassarah fi’l-Adyaan wa’l-Madhaahib al-Mu’aasirah; al-Nasraaniyyah min al-Tawheed ila al-Tathleeth by Dr Muhammad Ahmad al-Haaj; Muhammad fi’l-Kitaab al-Muqaddas by Prof. ‘Abd al-Ahad Dawood [available in English under the title Muhammad in the Bible]; Diraasaat fi’l-Adyaan: al-Yahoodiyyah wa’l-Nasraaniyyah by Dr. Sa’ood al-Khalaf; and Rasaa’il wa Manaaziraat by Shaykh Ahmad Deedat [Shaykh Ahmad Deedat’s booklets and videos are widely available in English].
You can also benefit from websites that specialize in this topic, such as the following (in Arabic):
4 – He should strive to debate in a polite and fair manner, and look at the one who disagrees with him in a compassionate manner. So he should be keen to guide him and strive to convey the truth to him, like a sympathetic doctor treating his patient. So he should not mock him or laugh at him, and he should not act superior or arrogant, rather he should call him to the way of his Lord with wisdom and beautiful preaching, and respond in the way that is better, as Allaah says (interpretation of the meaning):
“And argue not with the people of the Scripture (Jews and Christians), unless it be in (a way) that is better (with good words and in good manner, inviting them to Islamic Monotheism with His Verses), except with such of them as do wrong; and say (to them): ‘We believe in that which has been revealed to us and revealed to you; our Ilaah (God) and your Ilaah (God) is One (i.e. Allaah), and to Him we have submitted (as Muslims)”
Al-Qurtubi said in his Tafseer: It is permissible to argue with the people of the Book in the way that is better, in the sense of calling them to Allaah and pointing out the sound evidence and proofs, in the hope that they may respond to the call of faith, not by way of proving them to be wrong and treating them harshly. End quote.
Many Christians, when they hear the arguments of the Muslims and realize that what they were following is wrong, believe and are guided, and they succeed and prosper. Using good methods in calling people has a powerful effect in that regard.
We should re-emphasize at this point the importance of arming oneself with shar’i knowledge and studying this matter in depth, which has become a branch of knowledge in its own right nowadays, before indulging in it, because if you indulge in this matter before preparing yourself and studying enough about it, your weakness and incapability will backfire against the religion which you want to promote and defend.
Strive to make contact with people in your country who are specialized in this field, and whose religious commitment and knowledge can be trusted, to help you with this matter and guide you to what you need.
We ask Allaah to help and guide you.
And Allaah knows best.