Final answer:
The purpose of the dash is to highlight the most important point in the sentence.
Step-by-step explanation:
The purpose of the dash in the sentence 'Thus, we do have one official dogma—each American may think and express anything he wants' is to highlight the most important point.
The dash is used to place more emphasis on the content that follows it, in this case, the statement 'each American may think and express anything he wants'.
By using a dash, the writer is drawing attention to this important point and making it stand out in the sentence.