How well should you know electromagnetism before studying Antenna Theory
I haven't read the book from Balanis, so I can't say for sure at which level of depth it treats antennas (i.e., does it cover antenna design?).

Anyway, not knowing anything about wave propagation will hamper your understanding severely. Knowing Maxwell's equations (MEs) is just the starting point.

In particular, MEs is the basis for understanding transmission lines and then waveguides. Antennas are useless if you don't know how to convey a signal to/from them, and for that you need transmission lines and/or waveguides.

Moreover, antenna theory heavily involves wave propagation in free space, so if you didn't study wave theory, you'll be in trouble, unless the course you are going to follow focuses on mere qualitative analysis.

See the relevant article on Wikipedia, for example. It is almost entirely qualitative, and touches an incredible range of subjects.

Every one of that subjects can be developed quantitatively, and the theory can be quite complex.

Anyway, it seems you are following a college/university course, hence the best advice is ask your teacher or a student tutor (if they exist in your university system), or even students that already followed the course you are going to follow.