Religion was a great problem for the Tudors.

When Henry VIII came to the throne, England was Catholic.  Henry VIII changed the church so he could grant himself a divorce and marry Anne Boleyn - because he wanted a son so much.  Henry started the Protestant church in England, known as the Henrician Reformation. 

When Henry died in 1547, his son, Edward VI became King.  He was a strong Protestant.

When Edward VI died, Mary became Queen.  She was a very strong Catholic - so changed religion back again.

Elizabeth tried to find a balance between the two religions, hoping to stop the problems that religion had caused England.  As she tried to find a solution, this is often known as 'a middle way'.