“You are the salt of the earth, but if salt has lost its taste, how shall its saltiness be restored? It is no longer good for anything except to be thrown out and trampled under people’s feet.” (Matthew 5:13, ESV)
Christians lose their saltiness when they accommodate the culture in the name of contextualization. To contextualize is to remove unnecessary obstacles to the gospel (i.e., learn the language of the culture, lay aside extra-biblical preferences).
To accommodate is to stop preaching against the sins of the culture to grow a bigger congregation or to get more twitter followers and speaking gigs.