I let him know that a Christian can believe in fairies, if he wishes. While I wouldn't doubt the salvation of one who did, I may doubt his sanity. This is because Christianity doesn't come from what you believe (although that is part of the equation), it comes from who you know.
Let me back up a little to explain what I mean. The Bible teaches the Jesus Christ was pre-existent before He was manifest in human form. His claim about His identity was utterly unique. He maintained that He was the very source of life, saying a number of times, words like "I am the life." When someone repents and believes the gospel (that Jesus Christ died for his sins and rose on the third day), he places his trust in the Savior and comes to "know" God. Then God "seals" the believer with the Holy Spirit. Jesus Christ who Scripture says "is our life" then dwells within the believer. Colossians 1:27 says "Christ, who is in you," and in John 14:21 Jesus said that He would come to and would actually dwell within the Christian through the Holy Spirit.
Here now is the bottom line. If you have Jesus Christ, you have life. If you don't have Jesus Christ (through the new birth of John chapter 3) you don't have life. You are still dead in your sins, and justly under the condemnation of God. Here now are the pivotal verses--from1 John 5:12-13):
"He who has the Son has life; he who does not have the Son of God does not have life. These things I have written to you who believe in the name of the Son of God, that you may know that you have eternal life..."
That said, if you have the Son of God, then we are told that the Holy Spirit will lead you into all truth. Your theology in time will become "sound," and you will align your beliefs with those truths revealed in Holy Scripture. If the Old Testament says there was a literal flood (as did Jesus), the Christian cannot believe otherwise. If the Bible, in Job 26:7 says that the earth freely floats in space when "science" of the time said that it didn't, the Christian quickly sides with the Bible.
In the case of evolution, Scripture is very clear that God made man in His image (not as a primate). He made him and all the animal kingdom as male and female, and He gave them (and every living animal) the ability to reproduce "after their own kind," and not to evolve in time into other "kinds" or species of animals. We see the truth of all of the above both in the fossil record and in the creation that surrounds us.
So if you call yourself a Christian and you think that a belief in God and a belief in evolution are compatible, you will have to label yourself an “idolater.” This is because the god you believe in is not the God who revealed Himself in Holy Scripture. You have made up a false god and you are in violation of the First and the Second of the Ten Commandments. So how then can you call yourself a Christian? If I were you, I would quickly obey the admonition to examine myself and see if I was truly converted.