How does someone have a "lack" of belief in something they know about?

lets take Santa Claus for example...i can intellectually understand the concept of Santa Claus...now if i intellectually understand the concept of Santa Claus how do i have an intellectually "lack" of belief...i can either choose Santa Claus is real, Santa Claus isn't real, or i don't know if Santa Claus is real..."i don't know" is intellectually honest..."he is real" is a statement of faith..."he isn't real" is also a statement of faith...how about "i have lack a belief in him"...lack implies missing something...yet intellectually you aren't missing anything for a belief...