Why are there different denominations between Christians?
A denomination is a division and, yes, among those calling themselves "Christian" there are a great many divisions. Even among a particular denomination, there are divisions as evidenced by the Episcopals' arguments over their homosexual bishop, the United Methodists' arguments on where to allow homosexual unions, and the Catholics' arguments over women in the priesthood. I know of no group which has not experienced division within their ranks.
The fact is that division is bound to occur. Paul warned the Corinthians, "For first of all, when you come together as a church, I hear that there are divisions among you, and in part I believe it. For there must also be factions among you, that those who are approved may be recognized among you" (I Corinthians 11:18-19). There will always be those who will not stay with the truth of God's word. As a result, there will be divisions. There is only one God, but there are many men each with their own ideas concerning how they think religion ought to be handled. Division comes when men try to please themselves instead of pleasing God (Galatians 1:10). Their religion is useless because it places the commands of men above the commands of God. "Hypocrites! Well did Isaiah prophesy about you, saying: 'These people draw near to Me with their mouth, and honor Me with their lips, but their heart is far from Me. and in vain they worship Me, teaching as doctrines the commandments of men.'" (Matthew 15:7-9).
Just because divisions happen, it doesn't mean that we should accept them. Jesus stated that He founded just one church. "And I also say to you that you are Peter, and on this rock I will build My church, and the gates of Hades shall not prevail against it" (Matthew 16:18). Paul confirmed that only one church (or body) exists. "There is one body and one Spirit, just as you were called in one hope of your calling" (Ephesians 4:4). The description of that one church is found in the pages of the New Testament. Those following God's plan exclusively become a part of that one church.
Even in the early days of the church, Christians began to divide, but Paul gives the remedy for this disease. "Now I plead with you, brethren, by the name of our Lord Jesus Christ, that you all speak the same thing, and that there be no divisions among you, but that you be perfectly joined together in the same mind and in the same judgment. For it has been declared to me concerning you, my brethren, by those of Chloe's household, that there are contentions among you. Now I say this, that each of you says, "I am of Paul," or "I am of Apollos," or "I am of Cephas," or "I am of Christ." Is Christ divided? Was Paul crucified for you? Or were you baptized in the name of Paul?" (I Corinthians 1:10-13). Here then is the solution: Christians must approach Christianity with the same goal (the same mind) and using the same standard (the same judgment). When they do so, they will speak the same thing.
What should be our goals? "Jesus said to him," 'You shall love the LORD your God with all your heart, with all your soul, and with all your mind.' This is the first and great commandment" (Matthew 22:37-38). If we put pleasing God first on our list of priorities, then it doesn't matter what you think or what I think, it only matters what God wants. "And we have known and believed the love that God has for us. God is love, and he who abides in love abides in God, and God in him" (I John 4:16). To put this simply, it means we will be humble enough to submit to following God's commandments. "For this is the love of God, that we keep His commandments. And His commandments are not burdensome" (I John 5:3).
A natural result of a love for God is a love for our brethren. "Beloved, let us love one another, for love is of God; and everyone who loves is born of God and knows God. He who does not love does not know God, for God is love. In this the love of God was manifested toward us, that God has sent His only begotten Son into the world, that we might live through Him. In this is love, not that we loved God, but that He loved us and sent His Son to be the propitiation for our sins. Beloved, if God so loved us, we also ought to love one another. No one has seen God at any time. If we love one another, God abides in us, and His love has been perfected in us" (1 John 4:7-12). In this, we will strive to live in harmony with one another. It doesn't mean we will ignore each other's faults or compromise our God's commands for the sake of artificial peace. Instead, we will work with each other and strive for a common understanding. We will encourage each other to reach for the lofty goal of being Christ-like. "And He Himself gave some to be apostles, some prophets, some evangelists, and some pastors and teachers, for the equipping of the saints for the work of ministry, for the edifying of the body of Christ, till we all come to the unity of the faith and of the knowledge of the Son of God, to a perfect man, to the measure of the stature of the fullness of Christ; that we should no longer be children, tossed to and fro and carried about with every wind of doctrine, by the trickery of men, in the cunning craftiness of deceitful plotting, but, speaking the truth in love, may grow up in all things into Him who is the head -- Christ -- from whom the whole body, joined and knit together by what every joint supplies, according to the effective working by which every part does its share, causes growth of the body for the edifying of itself in love" (Ephesians 4:11-16).
Then, too, we need to have the same standard for judgment. God has only given man one system of belief (Ephesians 4:5). " Beloved, while I was very diligent to write to you concerning our common salvation, I found it necessary to write to you exhorting you to contend earnestly for the faith which was once for all delivered to the saints" (Jude 3). God told us what we need to do once for all people and for all of time. The Bible is our single standard from which we learn how to please God and by which we judge what is pleasing to God.
At times you will hear people urge others to "join the church of your choice." It is such a foolish idea. We need to join the church of God's choice, only then can we be one.