Fichte became one of the most influential founders of German Idealism after one of his books was published anonymously and people attributed it to Kant. Kant said he didn’t write it, but that whoever did was one of the only people to understand him (basically). The primary difference between them: Kant rejected Idealism. Fichte was unconvinced by Kant’s argument, holding that there is an unbridgeable gap between the world of objects and the world of ideas.
As much as he admired Kant, being an Idealist is a matter of character, and some people just do not have what it takes. An Idealist accepts the responsibility of the incredible freedom with which philosophy presents him.
In contrast, Fichte calls any belief in the world of objects “Dogmatism,” presumably because it cannot defend its belief in the world of objects. We are born with a need to believe the world of objects is deterministic and real, but only has essence when it is observed.
Idealism, on the other hand, which holds that we can only know our perceptions exist (not the objects themselves), and our self, which is not an object, but an ‘act’ known directly. The self is free since it is not a slave to the deterministic world of objects. Fichte argues that one should be able to explain the world exclusively extrapolating from self-evident phenomenological truths (described by Kant).
Dogmatism and Idealism cannot refute the other, but the former cannot explain freedom or the self, and arguments against Idealism reveal the ignorance and moral failure of those who make the attack.