We are comprised of all the makings of the universe which I believe is extremely rare among other species in the universe. As far as I'm aware we have infinite potential and that is why advancement is eternal. Power is control, whatever you can control and do is power. Attaining that power and control though continuous advancement to godhood and infinitely beyond is the never-ending journey.
I'd say "power" at the most basic level is to be able to do a working and observe how it manifests successfully into the physical and gives you results.
There are a lot of things that can indiciate power, because there is a lot that you can do with it. From healing ailments, removing bad karma, feeling calm and confident, having more energy and better health, to stuff like setting things on fire with pyrokinesis, moving things with your mind, or lowering/heightening your body temperature through control of the elements. The list goes and goes.
In the end it's all about how dilligent you are with your daily personal advancement and soul-cleaning, together with daily practice on whatever skill you want to develop. This goes from pyrokinesis to just simply having workings manifest properly. Practice is required for most people before big results are achieved.
in my opinion someone is strong when he is able to protect a satanic family. someone who is able to move forward for himself and his family. Satanists and gods are one big family. Development, meditation or other work are very important. Satanists' task is to become strong enough that the gods don't have to worry about us (if I wrote something incorrectly, I'm sorry, I'm not good at writing in English )