Why is the statement "all life is made of cells" an example of a theory? Explain

This is a theory because of several things;

A) We do not know if there are organisms we have yet to discover that are contrary to it.

B) Virus's are a stark and confusing parable for us. It is not a cell but contains DNA and Replicates.

Therefore this conjecture is just a theory until we can definitely prove the statement is correct, with science!