Asalam rahmatullahi wabaraktu. As was the method of the Prophet Muhammad(SAW) when he sent a sahabi to the Christians in Yemen, we should always begin with tauheed. Tell them Allah is one without sons, daughters or mothers. At this point the Christian may become defensive and mumble something about their "faith" in the trinity and/or divinity of Jesus. If the person is not completely brain dead and not averse to history; then you can add a few more things. Mention to them that the early Christians were divided as to whether Jesus was divine or human, whether he died on a cross and whether he taught a trinity or belief in one God. There was literally wars over these issues. It was not until the Council of Nice in 325 AD that the Roman Emperor Constantine(who was a five star kaafir) forced the followers of Jesus(Peace Be Upon Him) to adopt the belief of the trinity, the divinity of Jesus(Peace Be Upon Him) and the death on the cross. I might not hurt to add that this Council decided which books would make the final cut in the Bible they have today. There was a book called the Gospel of Baranabas where the author states that Jesus(peace be upon him) mentioned the Muhammad(SAW) by name, did not die on the cross and preached tauheed. Funny how the Roman empire made this book illegal and killed anyone in possession of it. Mention to them this history and the subsequent cover up. And if Allah opens their heart to the haqq then alhamdulilah, but you did your part.