Convincing people that true prophecy exists can lead to fatalism.
It does not really matter what people think is the source of true prophecy. The sheer existence of true prophecy can lead to the conviction that the future is already fixed and predefined.
Fatalism is generally the belief that events are determined by fate and that nothing can be done to change it's outcome. The future is already pre-determined by something over which humanity has no control.
Of course this is in conflict with the theory of free will, which allows people the freedom to choose their actions, which are not predetermined by any force other than their own will.
Fatalism may be consistent with a belief that events are caused by a determining principle or force in the universe, such as God who determines everything. Every event, good or bad, may be seen as part of a plan. Individuals must accept what happens to them, because it is what God willed to happen.
The possible existence of true prophecy might indicate that the future is already known but at the same time it could be an indication that the future is not fixed and can be changed. Since true prophecy allows us to know future events, we could prepare and alter or evade these events.