Full Idea
Advocates of the mind-body identity theory typically claimed that identity between particular mental states and brain states was contingent, until Kripke argued persuasively that identity is always necessary.
Gist of Idea
Prior to Kripke, the mind-brain identity theory usually claimed that the identity was contingent
Source
John Perry (Knowledge, Possibility and Consciousness [2001], §8.1)
Book Reference
Perry,John: 'Knowledge, Possibility and Consciousness' [MIT 2001], p.171
A Reaction
Kripke wanted to argue against the identity theory, but what he seems to have done is reformulate it into a much more powerful version (involving necessary identity).