Never say this again: "Well, I'm a Christian, so..."
There are a few reasons for this request. If you were to say, "Well, I'm a feminist, so I believe in equal rights for women," it might be okay, but even in this example you can see that it's redundant: you're just defining what a feminist is. You could just say "I believe in equal rights for women" and it would mean the same thing. Also, you don't believe in equal rights for women because you're a feminist, because you have this feminist label. You are called a feminist because you believe in equal rights for women. Similarly, when you say "Well, I'm a Christian, so..." you're mixing up cause and effect.
Unfortunately, you're not even being redundant if you say "Well, I'm a Christian." You're simply not communicating anything. Because the label Christian has no real meaning. You can hear that the word has no meaning when you listen to what comes out of different people's mouths after saying this phrase:
"Well, I'm a Christian, so I believe in the trinity."
"Well, I'm a Christian, so I don't approve of homosexuality."
"Well, I'm a Christian, so I am a member of the Republican party."
"Well, I'm a Christian, so I believe we need to do missionary work."
"Well, I'm a Christian, so I believe the Pope is infallible."
You couldn't get all Christians to believe in any one of those things. I'm not sure that an umbrella definition for Christian exists, and there certainly isn't a shared belief system, in spite of some apparent similarities. (The best you could say, and be universal, is "Well, I'm a Christian, so I have some belief having something to do with Christ." Maybe.) So when you use this phrase, what you're really saying is, "Well, my version of Christianity forces me to believe this..."