The Bible is a complex book. In fact, the Protestant version is composed of sixty-six books, written by a range of authors. There are many themes that run through the Bible, and there are many places in the Bible that can be points of confusion.
The Bible is also a book that has been interpreted in various ways for close to 2000 years. There are many writings about the Bible and traditions that have arisen from these interpretations.
In Western Europe, prior to the early 1500s, the Catholic Church was the only official Christian church. People who interpreted the Bible differently were called heretics and received terrible punishments if they were caught. In fact, the word "catholic" means "universal."
Churches, such as Baptist, Pentecostal, Calvinist, Lutheran, Methodist, and so forth, did not arise until Martin Luther, then a monk in the Catholic church, challenged some of the Catholic traditions of the time. For example, some priests had been selling God's forgiveness of sin. Luther challenged these traditions and pioneered the essence of the "Protestant" churches. "Protestant" comes from the word "protest."
Luther's successful challenge opened the door for churches to be established around a range of interpretations of the Bible. For example, the Pentecostal churches focus on the Book of Acts as a primary source for how a church should be. In the Book of Acts, the Holy Spirit comes down upon the disciples and they preach to people but each person heard the preaching in their own language.
Baptists differentiate themselves from other Protestant churches in part through a focus on the practice of baptism. Some churches baptize babies, and they may do this by sprinkling a small amount of water on a person's head. Baptists believe in full immersion baptism, following the way in which John the Baptist baptized Jesus (in the river Jordan) and also with a concern for the way the Apostle Paul describes baptism as a form of death, burial, and resurrection of the believer's soul. These churches don't baptize people until they are old enough to make that decision for themselves.
Quakers believe that revelation from God did not stop with the Bible. They are also one of the peace churches, a group of churches whose members have chosen to be non-violent. They don't go to war because they believe there is the "light of God" in everyone.
Catholics, Baptists, Calvinists, and so forth are all Christians, and the Bible is supposed to have a central role in their faith. They believe Jesus Christ was the Messiah promised in the Old Testament of the Bible and that Jesus lived, died on a cross, was buried, and then resurrected, which are the events described in the first four books of the New Testament (the Gospels). Beyond this essential belief, they differ in methods of worship and doctrine, based on differing interpretations of the Bible.