Magickal practitioners when they first start usually will hold the understanding that belief in Magick is required for it to work, or for it to affect you, but from what I found for my experience, and study this is simply not true. I believe this idea probably stems from media interpretation, though I’m not entirely sure. Magick is a process of creating change in conformity with your will, and the fundamental parts of the process that it seems to need in order to bring forth manifestation are intent, will, and action. The efficacy of this process is not dependent upon the belief of the individual, though belief can play an important part in magick.
Belief may not be necessary for it to work, but it can be used as a tool in order to focus your consciousness, and energy towards a specific goal, which can greatly help improve your magick. it will also give you the motivation to carry out the magickal processes effectively, and to its completion, while helping you not to self sabotage your own magickal working. Though it is also important to know that true belief does not need to be employed in order for you to get such benefits, and it can simply be the processes of suspension of disbelief.
No matter what somebody believes magick will affect them in the same way fundamentally, though individuals who do not believe in magick, and are not open to it will wall their consciousness, and energy from being influenced through the same methods in which are employed in casting magick. This does not mean they cannot be affected by magick, though it does mean that they are more resistant to such processes mentally, and spiritually, and because of this it can seem that they are immune, even though they’re not.
Magick is a process. One that is not dependent upon belief, but can be improved by it. Learning to be able to use your belief in a way that aligns with your goals can help you in many areas of your life including the practice of magick.