That's the title of this post by a Jordanian Christian. I think that his surmise that most of us in the US automatically equate Arab and Muslim is accurate.
Stereotyping is a handy way to turn other human beings into cardboard cutouts, debasing their humanity. It also lets us avoid the pesky demands of actually thinking. In any case, stereotyping people is a clear violation of Jesus' command that we love our neighbors as we love ourselves. (Read here to see the command in its context.) As Christians, that's our call whether our neighbors are Muslims, Jews, atheists, Buddhists, or Christians.
But to dismiss the commitment of others who confess Jesus Christ as Lord of their lives because they belong to a different ethnic group or because many members of that ethnic group profess a different religion is at the least, misinformed. Sometimes, it's hateful, certainly a sorrow to Christ.
Now, in fact, I must confess that I too have made the same automatic connection between Arab and Muslim, for which I ask God and my Arab sisters and brothers to grant me forgiveness.