How to resolve issue of circular definition for an `If` statement
One general rule of Turing completeness is that you need some form of conditional branching built into your model. A Turing machine, for example, can transition to a new state based on what it sees on the tape. Programming-language-wise, a while loop works, as does an infinite loop with a conditional break statement, or if and goto.

As such, if you have no form of conditional branching built into your model already, you can't create one out of nowhere—if your model isn't Turing complete, you'll need to add something else from outside to the model to make it so.

An "atomic" if statement will do the trick, certainly, as will something like Lisp's conditional. But whatever it is, it'll have to be implemented by the compiler or interpreter, rather than being created within the language itself.