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..."
The word forces in the rephrase above is my next problem. Again, look at cause and effect. Do you (to pick one of the above examples) not approve of homosexuality because you're a Christian or do you not approve of homosexuality for some other reason? If it's the latter, then just state your reason. If it's the former, then really this isn't a "reason" at all. It has nothing to do with reason. It has only to do with a somewhat arbitrary definition that you think you are following. To go back to the feminist example, you might say that you believe in equal pay for men and women because it is fair (and you could go on to explain why it's fair), but you wouldn't say "because that's what I think feminists believe." You wouldn't say "Because I was raised feminist" and leave it at that (at least, you wouldn't have to leave it at that).
You can truly see the fruitlessness of the "I'm a Christian, so..." phrase when two Christians who use it talk to one another. Listen:
CHRISTIAN 1: Do you think abortion is wrong?
CHRISTIAN 2: Well, I'm a Christian, so yes.
CHRISTIAN 1: But I'm a Christian too, and I don't think abortion is wrong.
CHRISTIAN 2: Then you must not be a Christian.
CHRISTIAN 1: But I am, and that's what we believe.
Now listen to a Christian and a Non-Christian for another version of fruitless conversation:
NON-CHRISTIAN: Do you believe we should spank our kids?
CHRISTIAN: Well, I'm a Christian, so I don't believe we should.
NON-CHRISTIAN: What about "Spare the rod, spoil the child"? I thought that's what Christians believed.
CHRISTIAN: Maybe some do.
NON-CHRISTIAN: Personally, I think that children need a harsher form of discipline than "time out." Spankings seem to be more effective.
CHRISTIAN: Well, you say that because you're not a Christian.
Of course the Non-Christian in the above dialogue approves of spankings not because he's "not a Christian" but because he actually has a reason, and he's able to give it. The Christian in the above example, however, can't provide any real reason for himself, and -- what's worse -- he ignores the Non-Christian's actual argument, pointing instead to something that isn't a reason at all.
Just to clarify, I'm not saying that Christians individuals don't have real reasons for believing what they believe. Most do (or at least I hope they do). Most can provide arguments that don't simply fall back on the answer "because I'm a Christian." But whether real reasons exist or not, the phrase should go away. Then the real reasons can emerge from behind the clouds. And those people without real reasons, who rely on the phrase, might begin to actually think for themselves.