In order to look at the three important ideas of cell theory, one must first understand what cell theory is. Cell theory was purposed by Theodor Schwann and Matthias Schleiden in 1838. Amazingly, cell theory still holds true today as the principle that drives all of cell biology. The three main and most important ideas of cell theory are:
1) All living things are made from one or more cells.
2) Cells can only come from pre-existing cells.
3) A cell is the smallest form of life.
Interestingly, there are a few examples of organelles, such as mitochondria in animal cells and chloroplasts in plant cells, that are self-replicating, but are considered an exception to cell theory because they replicate within the cell that they reside in. Hope this helps!