The Sin Nature is destroyed in believing in the death and resurrection of Jesus Christ.     We are then Born Again as New Spirit beings In Christ.     

Paul is wondering if the believers understood what Water Baptism meant.     Which is, that we ourselves are immersed into His death so that we arise to newness of life in His resurrection.     

Without that understanding, baptism is simply a ritual.     Hence, the work of baptism is immersion into what it means to receive Christ Jesus as Saviour and Lord.