Here are the three main tenets of cell theory:

1. All living organisms are composed of one or more cells. This is the statement you mentioned.

2. The cell is the basic unit of structure and organization in organisms. This means that cells are the fundamental building blocks of all living things.

3. All cells arise from pre-existing cells. This means that cells don't spontaneously appear, they come from other cells dividing.

So, the statement "all living things are made of cells" is not only a part of cell theory, it's one of the most fundamental parts.