Do Christian Denominations Prove Christianity Wrong?
There’s a curious sentiment from people of different belief systems that I have heard on occasion. The comment goes something like this: There are so many Christian denominations, therefore Christianity can’t be true nor is it from God. If it were really true, then it would be completely united without division. Mormons, Jehovah’s Witnesses, Muslims etc have all voiced this opinion to me. Before we address this comment, I would like us to consider an obvious issue with these groups. They have had their own schisms, theological differences, and sects/splinter groups within their belief systems as well! This is a pot calling the kettle black scenario.
In the Muslim faith there are the Shiites battling the Sunnis, that being figuratively and literally. Now the Baha’i Faith, another monotheistic religion founded by Bahá’u’lláh in 19th-century Persia  lays claim to have superseded Islam as the continued revelation of the faith. Of course Muslims reject this notion and instead cling tightly to Muhammad’s dictated writings. With the Mormons, they have close to 200 splinter groups charged to their account. Some of the groups include the Short Creek Community, Latter Day Church Of Christ, School Of The Prophets, True and Living Church of Jesus Christ of Saints of the Last Days, and the Blackmore/Bountiful Community. 
The Jehovah’s Witnesses had their schism in 1917. Their second president JF Rutherford, seized the presidency from the advisory board (and editors of their publications) that the founder Charles Taze Russell had set up before his death in 1916. At the time of their founding in 1879 until the schism, they were known simply as “Bible Students”. Rutherford changed the name of his branch of Bible Students to Jehovah’s Witnesses in 1931. He did so as to distance his group from the other Bible Students that were still holding on to the teachings of Russell. This group -also known to some as Russellites- still exists today but many Jehovah’s Witnesses are unaware of their existence.
How To Address The Problem
How can we as Christians address the charge that Christianity is so divided -and therefore wrong- because of all it’s Christian denominations? I’m glad that you asked. There’s an interesting story in the gospels that I would like us to consider. In Mark 9:38-41, the disciples were concerned about individuals who were casting out demons in the name of Jesus but weren’t in their group.  Jesus diffused their complaint by saying: “yo fellas, they’re on our team, they’re doing the will of the Father…chill out”. Ok that’s my paraphrase. If Jesus were to teach and relegate exclusivity of his power -and salvation that he gives for those who ask- to one group of people, this would have been a fine place to do so. The reality is, everyone who calls upon the name of the Lord shall be saved (Romans 10:13). This promise is not just for “good” Baptists and Pentecostals.
There can be no doubt that denominations have been a source of “infighting”. Understandably, no one is right on every doctrine or theological position (some Bible verses/passages are univocal while others are equivocal) but after all, we still see through a glass darkly.  Despite such differences, denominations can serve a purpose for the good. Here’s what I mean. Each denomination and church has a mission statement to carry out their objectives, ultimately to walk out the great commission and subsequent discipleship. Some have a huge focus on missions work, others on church planting, while others yet may have a strong emphasis on family.
The cliche’, “different strokes for different folks” I believe applies here. No one church or denomination is right for everyone, as people’s needs, skills, personality, approach etc is different. Just as individuals make up the different members within the body of Christ, which serve different functions, the variety of Christian denominations can serve such a purpose as well. Bring in the military. Within the military there are the various branches such as the Army, Marines, Navy, and Air Force. These branches have many different types of units whether Airborne, Tankers, Snipers, Infantry etc yet they have one objective, to defeat the enemy. Christians can spend time debating -while in the battle- about which unit is most effective, or they can take aim and fire the weapon that is already in their hand.
3. (NASB) Mark 9: 38-41 John said to Him, “Teacher, we saw someone casting out demons in Your name, and we tried to prevent him because he was not following us.” But Jesus said, “Do not hinder him, for there is no one who will perform a miracle in My name, and be able soon afterward to speak evil of Me. For he who is not against us is for us.
4. (King James Version) 1 Corinthians 13:12 For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known.