In short: The threshold is not a part of the Naive Bayes algorithm A Naive Bayes algorithm will be able to say for a certain sample, that the probability of it being of C1 is 60% and of C2 is 40%. Then it's up to you to interpret this as a classification in class C1, which would be the case for a 50% threshold.
Text Classication using Naive Bayes Hiroshi Shimodaira 10 February 2015 Text classication is the task of classifying documents by their content: that is, by the words of which they are comprised. Perhaps the best-known current text classication problem is email spam ltering: classifying email messages into spam and non-spam (ham). 1Document models.