Regarding the true mark of Christianity, the risen and exalted Lord Jesus did not leave it up to us to figure it out. Rather, he left an unmistakable mark by which his followers would be identified through the ages.
By this all people will know that you are my disciples, if you have love for one another. (John 13:35)
However, through the centuries the followers of Jesus have not always been distinguished by this mark: our love for one another. Instead, we have shed blood, dehumanize fellow human beings, and even persecuted those with whom we disagree–all in the name of Christ.
But there is one true mark of Christianity: it’s our observable love for one another. It is what will lead unbelievers to say: “Look at those people, see how the really love one another! You know, I think I’ll give a listen to what they are saying about all this Jesus stuff”