Before replying to this question, I would like to say how impressed I am with my brother’s concern to call people to Allaah, using the developments in computer science to further the mission of the Prophets and Messengers (peace be upon them). May Allaah reward him with the best reward, and record him among His righteous slaves. With regard to the question you asked:
The best way to call this man and everyone else is that which is indicated in the Qur’aan, where 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)”
“Invite (mankind, O Muhammad) to the way of your Lord (i.e. Islam) with wisdom (i.e. with the Divine Revelation and the Qur’aan) and fair preaching, and argue with them in a way that is better”
And the command which Allaah issued to His Prophet (peace and blessings of Allaah be upon him) when He said (interpretation of the meaning):
“Say (O Muhammad): O people of the Scripture (Jews and Christians): Come to a word that is just between us and you, that we worship none but Allaah (Alone), and that we associate no partners with Him, and that none of us shall take others as lords besides Allaah”
[Aal ‘Imraan 3:64]
-Tell him about the innate nature of man (fitrah) which predisposes man to believe in Tawheed or the Oneness of Allaah, and how the doctrine of trinity contradicts that innate human nature and common sense.
-Explain to him the status that ‘Eesa (Jesus – peace be upon him) and his mother have among the Muslims, and that he is the slave of Allaah and His Messenger, and His Word which He bestowed upon Maryam (Mary), and a soul created by Him.
-Explain to him that the religion of Allaah is the same from Adam to Muhammad (peace and blessings of Allaah be upon him), and that it is Islam. Tell him that the differences are in the laws and that Muhammad (peace and blessings of Allaah be upon him) is the Seal of the Prophets (i.e., the last of them) and that his Book abrogates the rulings which came before and confirms the true beliefs and stories that were mentioned in them.
-Answer his questions calmly, and provide him, if necessary, with some of the books and tapes by Shaykh Ahmad Deedaat.
Answered by Shaykh Dr. ‘Abd al-Rahmaan al-Qaadi, Imam Muhammad ibn Sa’ood University, al-Qaseem Campus
You could also send him Question No. 2690, entitled A serious dialogue with a Christian from this site.