"Christianity" includes hundreds of different denominations that disagree with one another in name, organization, doctrine, worship, and plan of salvation. Such a situation clearly constitutes division, not unity. Yet all the denominations claim to be Christian and to follow Jesus Christ.
John 17:20-23 - Jesus prayed for all who believe on Him to be one as He and His Father are one. Jesus, the Head of the church, prayed for unity. Denominations claim that He is their head, yet they practice division!
Divisions in the church are a result of sin. Ever since Babel, mankind has been cursed with a sectarian spirit. Even the church in C1 Corinth, under the influence of the blessed apostle himself was riven by sectarianism. You'd have thought they'd all unite as a body under such infallible Scriptural teaching, but no. "I am of Paul" cried some, "I am of Apollos" shouted others, and still others "I am of Cephas".
How does Paul deal with such sinful division? In chapter 1:10-28 he appeals for unity under the message of the gospel.
I appeal to you, brothers and sisters, in the name of our Lord Jesus Christ, that all of you agree with one another in what you say and that there be no divisions among you, but that you be perfectly united in mind and thought.(v10)
Yet later in the same epistle in verse 11:19, Paul concedes that divisions, sinful though they are, serve a purpose in the mysterious providence of God. These divisions serve to show "who have God's approval."
I thank God that I baptized none of you except Crispus and Gaius, 15 that no man should say you were baptized in my name. 16 Now I did baptize also the household of Stephanas; beyond that, I do not know whether I baptized any other. 17 For Christ did not send me to baptize, but to preach the gospel, not in cleverness of speech, that the cross of Christ should not be made void. (1 Corinthians 1:14-17).
No Christian has perfect knowledge and understanding. If we did, we would be as God. We may be right or wrong in some of our beliefs and still be in fellowship with God and with one another. This does not mean we can be right or wrong about every belief or practice. God is clear about that. There are many divisions in the body of Christ because some believe that they must separate from everyone who does not believe exactly as they do. This is wrong and it must be corrected if there is ever going to be the unity in the body of Christ that Christ prayed for and that we are to endeavor to maintain. All Christians have a responsibility in this matter. We can either be working with those who promote unity in Christ or among those who cause division in the body of Christ. When we cause divisions we are to be pointed out and avoided (Rom. 16:17). When we promote division we are carnal and behaving like mere men (1 Cor. 3:3). Heresies (self-willed opinions that lead to division and the formation of sects) are listed as a work of the flesh that precludes one from inheriting the kingdom of God (Gal. 5:19-21). Titus, a New Testament evangelist was instructed to reject a divisive person after the first and second admonition (Titus 3:10).
God purposed in Himself to gather together in one, all things in Christ (Eph. 1:9,10). Jesus Christ prayed that all who would believe in Him would be one with God, one with Him and one with one another (John 17:20-22). The apostles pleaded with the church in the city of Corinth to all speak the same thing, and that there be no divisions among them (1 Cor. 1:10). The apostles also made a plea for the church in Ephesus to endeavor to keep the unity of the Spirit in the bond of peace. There is one God, one Lord, one Spirit, one faith, one baptism, one body and one hope (Eph. 4:3,4). In order to be one with God and one with Christ we must be willing to believe in and practice the teaching of Jesus Christ. He is the only way to God (John 14:6; the only Savior (Acts 4:12) and the one Mediator between God and men (1 Tim. 2:5). As Christians, we are required to accept the "essential" doctrine of Christ. "Essential doctrine" would include whatever Jesus and the apostles equated with (made equal to) salvation and stated as commands, charges, orders, mandatory and/or exclusionary.