Why was Freedom of Religion added to the First Amendment?
Each is a form of personal expression.
The first amendment is there because despotic government use controls on speech and religion as a way to control its citizens. In England everyone was expect to be a member of the Church of England. Both the Pilgrims and Puritans railed against that idea and even 150 years later it was an accepted American ideal that you worshipped as you pleased and the government had no say in such matters.
The freedom of speech portion was particularly important to Americans who desired to express themselves in the press and other forms of print. Britain had tried, and failed, to control Americans in this matter. The new country understood that a good democracy allows people to speak their mind freely.