Here's how I would say it:

Knowledge is information of which you are aware. This includes evidence, reasoning, etc. that you have encountered and assimilated into your memory.
A belief is knowledge that you hold to be true. Some beliefs are backed up by evidence and logic, some are not. The former are rational beliefs, the latter are irrational. Note that this does not mean that the latter are necessarily untrue, it simply means that they cannot be proven rationally.
Evidence is information about the world that can be directly observed. This generally includes anything that can be measured or tested.
A theory is an explanation of the aspects of the world that cannot be directly observed. Because of this, a theory is never as "certain" as evidence is, it can only be more or less likely to be true. The two primary things which affect whether a theory is likely to be true are logical correctness and consistency with the evidence. If a theory violates the laws of logic, it cannot be true. Furthermore, in the areas where the theory touches on aspects of the world which can be directly observed, it must accurately describe those aspects, or it cannot be true (i.e. it must be consistent with the evidence).

In other words, the most you can say about a theory is that it is logically correct and consistent with the evidence. The more evidence you have which is consistent with the theory, the more likely it is to be true, but this can never be proven beyond all doubt (which is why courts only require the jury to be certain beyond "reasonable" doubt).

Socionics, therefore, is a theory. In my opinion, it is a logical theory, and consistent with the available evidence. However, there is not as much evidence available in this field as there is in, say, physics or chemistry. This is a problem for psychology generally, not just socionics specifically, because both deal with the inner workings of the mind, which are not directly observable.

To put it all together, socionics is a theory which I believe to be true, because I have knowledge of much relevant evidence and logic, and have concluded that it is consistent with both.