I used to think this was actually being dogmatic, but when I actually wrote that the other day I decided to check myself, and it doesn't appear to be the case, however I'm fairly certain there is a word for this behavior, this deference to select authority (e.g. someone doesn't believe in climate change regardless of the scientific consensus but then some radio personality he/she listens to starts espousing the danger and suddenly the person then believes in climate change).
capricious - subject to, led by, or indicative of a sudden, odd notion or unpredictable change; erratic:
You could say that the individual shows a capricious discipline in the face of authority.
There are a couple options. First, the person might be intelligent enough to defer to an authority. Vice versa, the person might be an idiot who defers to someone who claims to be an authority, though that "authority" might actually be a propagandist.
I'm not sure what the best word for this is. Such a person could be called fickle, though that's a little lame. Conservatives sometimes speak of wishy-washy liberals, though that's really not the best choice, either.
In general, you're simply describing a type of propaganda. In fact, John Clifford just mentioned it: appeal to authority.
A person who falls for this type of thing could be loosely called a sucker or simply one of the sheeple.
If the authority figure is a sophisticated user of mass media, then the scapegrace is succumbing to a cult of personality. The authority figure is an expert at getting people to roll over
But this puts the onus on the authority figure.
If you want to place the fault squarely on the one changing allegiance, then they are a spineless jellyfish.
Spineless, 2. lacking resolution; weak and purposeless.
That's a fox in a boar's clothing.
It's a play on the old idiom about wolves and sheep. In politics those who never flip flop are boars. Those who do are foxes. Being a fox allows you flexibility. But being known as a fox will lose you votes from both sides. So smart foxes pretend to be boars. For as long as they can.
I would call such a man "POTENTIAL CONVERT". Men do change in situations like these when they abandon their age-old cherished views or dogmatic notions.
It sounds like you're asking about a loyal follower, as in "He is a loyal follower of Smith, whose opinion rests solely on what Smith has to say about the topic."
You might prefer the adjective mindless, which implies someone who doesn't consider a position before adopting it. E.g. "Smith's mindless followers espouse whatever opinion he offers, regardless of its logical fallacies or salacious nature."
The problem with many responses here is that they are loaded terms. This is ironic in the context of an inquiry about dogmatism. A neutral term would be acolyte or disciple. They are not necessarily pejorative terms, even though one might conclude that being an acolyte or disciple is not healthy.