Does anybody know what is the simplest possible theoretical programming language that is capable of doing all computing tasks? I assume it to be a language with just the following two instructions and the ability to create variables (eg a, b, c ... ) but maybe that is optimistic:

Let a = b
if a < b then go to instruction x

I assume Turing had a lot to say on this subject ...

I have a good reason for asking this which I would rather not go into at this point. All help greatly appreciated!