The proposition consists of two parts: Part 1: In order to produce... This introduces the subject by explaining that the lights can operate only through the letters. Part 2: The letters constitute... This explains what the letters are.
Part 1: In order to produce an actual effect, all of the supernal lights... The entire purpose of the wisdom of the Kabbalah is to understand the particular root of each of the different phenomena we find in the world. Anything that can exist without being dependent on something else must have its own root.