Why do we have different denominations?
Here's the answer:
The denominations have different beliefs on certain issues.
Many of the different denominations in Christianity (Presbyterian, Baptist, Methodist, Nazarene, etc.) came about because of something that happened in the 15th century. At that time, a Roman Catholic priest named Martin Luther broke away from Catholicism after he wasn't able to fix some wrong things in the church. This was called "the Reformation." Since the beginning of the Reformation, many different denominations have appeared.
Christian people usually attend different churches and denominations because they're comfortable with the people they meet there, the type of preaching, or the style of worship. Different churches offer lots of variety in music and worship styles, and this is fine. But all denominations that claim to be Christian must be united in one thing—truth. The only source of truth is the Bible, God's Word to us.
Whatever denomination our families attend, we're still to love other Christians in different churches, and to be "one" with them in loving and obeying Jesus.
"... We are many persons. But in Christ we are one body. And each part of the body belongs to all the other parts" (Romans 12:5).
"You are the body of Christ. Each one of you is a part of it" (1 Corinthians 12:27).
"There is one body. There is one Spirit. You were appointed to one hope when you were chosen. There is one Lord. There is one faith and one baptism. There is one God and Father of all. He is over everything. He is through everything. He is in everything" (Ephesians 4:4-6).