Someone who is dogmatic has arrogant attitudes based on unproved theories. If you dogmatically assert that the moon is made of green cheese, you'll just get laughed at.
The most basic definition of the adjective dogmatic is that it is related to dogma — doctrines relating to morals and faith — but what it has come to mean is attitudes that are not only based on unproved theories but are also arrogant in nature. The root of dogmatic is the Greek word dogmatikos. A synonym of dogmatic is "dictatorial" and because there are religious associations to the root word dogma, someone who is dogmatic tends to "pontificate."