Calling Christians to Islam
22-6-2002 | IslamWeb
Question:
Dear Brothers. I have several friends who are Christians, whose I would like to convert to Islam, but I am not sure how to approach the issue. I have already proposed the idea to them, but I might have driven them away from it just a bit. Please advise me on what to do.
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad 
is His slave and Messenger.
The first thing you should try to convince them about is the true monotheism; i.e. the Oneness of Allah. The Prophet said to Mu'aath 
when he sent him to the Christians and Jews in Yemen: "You are going to a nation from the people of the Scripture, so let the first thing to which you invite them, be the Oneness of Allah." In fact the most important difference between us and the Christians is that we Muslims say and believe that none is worthy of being worshiped but Allah Alone. There are no associates with Him. He is the Self-Sufficient Master, Whom all creatures need, but He neither eats nor drinks. He begets not, nor was He begotten. And there is none co-equal to Him.
Whereas the Christians say that Jesus ('Eesa) 
is the Son of God. They consider him as a God with God. They say that God is one of three. Glorified and Exalted Be He from the great falsehood that they say! (i.e. forged statements that there are other gods along with Allah, but He is Allah, the One, the Self-Sufficient Master, whom all creatures need. He begets not, nor was He begotten, and there is none comparable or co-equal unto Him).
There are many pieces of evidence proving that Allah is One and Alone. Allah Says (what means): {Allah has not taken any son, nor has there ever been with Him any deity. [If there had been], then each deity would have taken what it created, and some of them would have sought to overcome others. Exalted is Allah above what they describe [concerning Him]!} [Quran 23: 91]
So, Dear Brother, do your best to explain to these men the true monotheism through pieces of evidence from what can be perceived, seen, or heard. Everything is an evidence to the Oneness of Allah. Remember also that Allah Says (what means): {Verily you (O Muhammad) guide not whom you like, but Allah guides whom He wills. And He knows best those who are the guided.} [Quran 28: 56]
If you feel some shortcoming in explaining the truth because of your little knowledge, get somebody who is more knowledgeable to explain this to them. You can also get help from some books that deal with the faith of the Christians and the contradictions that are in it.
Know, however, that Allah has warned Muslims from befriending disbelievers. Allah Says (what means): {O you who have believed, do not take the Jews and the Christians as allies. They are [in fact] allies of one another. And whoever is an ally to them among you - then indeed, he is [one] of them. Indeed, Allah guides not the wrongdoing people.} [Quran 5: 51]
For more benefit on calling non-Muslims to Islam, please refer to Fataawa 84349, 91563, 21363, 30315 and 90486.
Allah knows best.