First, let’s deal with the pronunciation. The ‘e’ in both words is pronounced like the ‘ee’ in ‘feel’, ‘peel’, and ‘heel’. The final ‘ism’ is like the ‘ism’ in ‘prism’, ‘materialism’, and ‘cronyism’. The stress in both cases is on the first syllable. ‘Theism’ comes from the Greek ‘theos’, and ‘deism’ comes from the Latin ‘deus’. Both words mean ‘god’. There are two different views about the creation of the world and God’s role in it. Deism is the belief that God created the world, and once He completed the task, He decided to call it a day! ‘Theism’, on the other hand, is the belief that God not only created the world, but also continues to play an active role in the running of it. In other words, He intervenes in the life of human beings; He is not a mere observer. This is just one of the differences.