One thing that is supposed to be a Christian attribute is humility. So when someone makes it clear that he or she isn’t Christian, yes at first glance it’s irritating to have that person pose as an expert on how Christians should act.
On the other side of the coin, Christians are folks who tell themselves the story of the Good Samaritan. In common parlance that would be the Good Illegal Alien in the role of the guy who helped a person who had been robbed by thieves, wounded and left beside the road. A traveling salesman and a rabbi – read minister or priest – both detoured around him before a foreigner (Samaria was way the wrong side of the tracks in those days, so the Samaritan would be about the same as an ‘illegal alien’ today) tended to his injuries, schlepped him to the nearest Holiday Inn and paid his room and board a week ahead.
So when a professed Christian gets upset about the influx of illegal aliens, don’t call that person names, just ask for a rendition of the Good Samaritan parable, but using “illegal alien” in place of “Samaritan.” Let the person digest it later, in peace and quiet. Honey wins friends quicker than vinegar, so keep your words sweet and let the misguided Christian do his or her own math.
Both of you will wind up happier when name-calling and shaming aren’t obvious, and you’ll likely have a far greater impact on the world than you would by throwing righteous disgust as the first punch.