Well why don't we ask what would Jesus do at this point? How would he confront the non believer? How would He confront someones obvious bashing of christianity? Would he walk away or would He speak up for Himself? What would Jesus say to the Muslim, Bhuddist, Shintoaist, Taoist, etc?
In today's political climate it's perfectfully fine to bash a christian. Don't say anything that might offend another group. Demand that any other christian speak softly for fear of upsetting someone. Demand apologies from christians for speaking out that christianity is a good thing.
Yes, sometimes christians go overboard, it is much better than not saying or doing anything, hiding hoping someone else will speak up and take the heat..
Jesus never backed down when asked and why should we? Are we perfect in everything we do? Obviously not, we are human just like the people who always have the need to bash, put down or try to correct what they think is bad behavior in a christian.
I think the point of the story regardless of being an expert in ruminant scatology is, you may think you are smart but God uses the simplest of things (fecal matter at this point) to show us that we really don't know that much about anything.