Baptists started with two main churches. One began in 1609, when John Smyth led a group of separatists to the Netherlands to start the General Baptist Church with an Arminian theology. The other began in 1616, when Henry Jacob led a group of Puritans to starts a church with a Calvinist theology. To this day, Baptists still are divided, with the majority believing in Arminian or Calvinist theology. There are also a few that follow Full Preterits beliefs.
As you may have guessed, Baptists focus on the importance of Baptism. John Smyth wrote a tract titled "The Character of the Beast" also known as "The False Constitution of the Church." It states that infants are not to be baptized because they are not old enough to comprehend salvation. To become a member and obtain salvation, a person must publicly profess their faith in Jesus Christ, followed by a water baptism. I mentioned before that God doesn't care about your church membership or your literal baptism, but about your heart and inward intentions. Although the Bible does say you should be baptized, it is not necessary for salvation.