"I am sure."
It can be used and would not be wrong. Although, it is not the most common use. It would mean that you are always sure, you never have doubts.
From another site reference: SER is used for permanent characteristics ("Soy enfermo" may mean you have AIDS and it won't change). ESTAR is used for temporal characteristics ("Estoy enfermo" means that you are ill in THAT moment, but will be health in the upcoming days).
Two years late, sorry, but maybe for somebody else
Soy means a permanent condition, as in you are always sure. It is a core part of your existence (probably not what you're going for now is it).
Remember, when it's how you feel and where you are, always use the verb estar!