Which comes first? Grammar or language?

This seems like a simple confusion of a polysemous concept: grammar. Grammar has several meanings when used both by specialists and non-specialists.

1. A set of rules determining how language is used to be acceptable (often in common parlance including things like spelling and stylistics). This is the subject of what is sometimes called 'prescriptive' grammar or 'pedagogical' grammar.

2. A book describing the rules mentioned in 1.

3. The structure of language that determines how it is used. This structure is learned by all speakers without (much) external intervention and is largely subconscious. This structure is the subject of study by descriptive linguistics.

By definition, grammar in the 3rd sense is the same things as language and therefore, one cannot precede the other.

However, the 'grammar' in the 1st sense does come after language in the sequence of acquisition. In fact, it is mostly a set of stories about language that more or less closely correspond to the actual patterns found in how language is actually used.