It's hilarious and tragic at the same time (How's that for duality?) that we can profess and purport the tenets and teachings of Jesus yet be so alienating with our attitudes.
We see at the inception of Christianity, the first instance of denominational barriers and distinctions. In the Gospel of Mark (9:38), John tells Jesus that he and the other disciples had criticized an individual for not following them. Yet this man was plainly following Jesus himself because God was casting out devils through him (9:39).
Paul had to deal with this in his ministry as well. In his first letter to the Corinthians (chapter 1), he criticized several people for mislabeling their faith by saying they followed other people instead of simply following Jesus. He asks this incisive question: "Is Christ divided?" (vs. 13). Later on in chapter three he speaks of it again, calling this divisiveness "carnal"—pertaining to our old nature. That's how we lived before we met Jesus. In his letter to the church in Rome (Romans 8:7), Paul says that the "carnal mind" is unable to think along God's lines (loose translation). He literally says that it (the carnal mind) is "enmity", or enemies, with God! How many of us might be born again but have not taken the steps necessary to renew our minds to God's truth? The end result is disunity and division. "These things ought not so to be." (James 3:10) Could it be that the "carnality" and "double-mindedness" spoken of in the New Testament are the reason that there have been so many rifts and splits in our churches?
I understand that many Christians view their brothers and sisters from different denominations with love and tenderness but the world doesn't realize this. They see the labels of "Baptist" and "Lutheran" and "Presbyterian", and because they don't believe in God in the first place, they think that we're just playing church and throwing words and concepts around for the fun of it. "Why can't Christians agree on anything?" This perception on behalf of the world will only change when we as Christians make the effort to show the kind of love to our brothers and sisters that Jesus exhibited. Jesus says that everyone will know that we're His disciples if we have love for one another (see John 13:35). And this "judgment" of love and mercy and forgiveness must begin in the church if we are ever going to win the world for Jesus.
"For the time is come that judgment must begin at the house of God..." (1 Peter 4:17a)