Why are there so many different churches? Sometimes churches of two or even three different denominations are with a couple blocks of each other. How did this happen?
For its first ten centuries the church mostly was one denomination. In AD 313 Emperor Constantine made Christianity legal and the "favored" religion of the realm. Fast forward 700 years.
The eastern half of what was once the Roman Empire (the Byzantine Empire) was more influenced by the ancient Greek culture and, began to slowly take on it’s own identity in style and even some doctrines. Then in 1054 it culminated when there was an official split and even mutual excommunications of West and East.
In 1517 the Protestant Reformation began as Martin Luther and others called out certain practices that had corrupted the church, such as selling certificates called “indulgences” to supposedly reduce years of Purgatory (a doctrine also rejected by the Reformers as not biblical).
The main distinction of the Protestant church, however, was a commitment to five principles, Scripture alone, faith alone, grace alone, Christ alone, God's glory alone.)
That is, only the Bible is ultimately authoritative for our beliefs, not the church or traditions. The church must always go back to the Bible to test its beliefs and practices. The second distinction, faith alone, means that salvation is by faith in Christ alone, not earned or helped by my good works (Ephesians 2:8-9).
Within the Protestant church as leaders began to examine the Scripture many questions of theology began to be addressed. With the political breaks from Rome that followed, there was a lot more freedom of different church groups such as Anabaptists, Puritans, Presbyterians, Methodists, Quakers and others, to re-examine the Bible . The Church of England had already broken away politically from Rome under Henry VIII, and this led to England and Scotland becoming a center for later Protestant thought.
The American colonies were also a natural place for the freedom of religion to continue to flourish. To name a few examples:
The Pilgrims were separatist Puritans who had fled previously to Holland, and then to America for religious freedom.
Later, Rhode Island was literally purchased from the native Americans in 1636 by Roger Williams, called the father of American Baptists, to be a haven for various denominations and freedom of religion.
The Quakers under William Penn settled much of Pennsylvania, where they too, found freedom to worship as they believed Scripture taught.
John Wesley, who preached tirelessly in England to spread the gospel, was the founder of Methodism, which spread rapidly in America in the following years through revivals during the Great Awakening.
The US Constitution protects the right to the "free exercise" of religion and that freedom has led to a proliferation of different denominations and of course non-denominational churches in more recent decades.
Most denominational differences do not involve essentials beliefs of the faith, such as the Trinity, the Deity of Christ, His death for sin and His resurrection, salvation by faith in Christ alone, the inspiration of the Scripture, and a few more.
Often the differences have to do with the way a group of believers chooses to organize (independent churches or a hierarchy?), baptism, minor doctrinal matters, or even style and overall vision or strategy for missions.
But all true churches that hold to the essential beliefs of Christianity are brothers and sisters in Christ!