“All that mitzvah that I am commanding you today – you must guard and keep it.”
The grammar seems not to fit here, and it should say “All those mitzvot” instead. However, this teaches us a rule. A person who started a mitzvah must finish it, because a mitzvah is ascribed to the one who finishes it. That is why the Torah said, “All that mitzvah” – and not part of that mitzvah.
Another way to understand this is to connect it to the prohibition discussed just prior – to distance one from idol worship. One who denies idol worship – which is defined as ascribing spiritual powers to any physical object or spiritual entity, as existing on its own, without being sustained and empowered by the Creator of all things – such a person is considered as if he observed perfectly the whole Torah.
Yet another way to understand “all that mitzvah” is to say that we are talking about the study of Torah. That is why it says, “All” – to include the Talmud and other oral teachings, which were later transcribed into writing. Then “guard and keep” means “learn and do and be careful not to forget what you learned.”
Art: Paul Seignac - Learning To Count