Sentence:  The congregation perishes when it is no longer possible to replenish that core from the neighborhood ; moreover , residential mobility is so high in metropolitan areas that churches have to recruit constantly in their <WSD>core</WSD> stratum in order to survive ; they can lose higher - and lower status members from the church without collapsing , but they need adequate recruits for the core stratum in order to preserve economic integration . 
Meaning of the word marked with <WSD> tags:  a small group of indispensable persons or things