This is from Dr. J. Vernon McGee's commentary on Luke chapter 15:
 
"Somebody asks,  'Do you believe that if you're once saved you're always saved?'  Yes.  Somebody asks,  'Do you believe that a Christian can get into sin?'  Yes.
 
" 'Can a Christian stay in sin?'  No.  Because in the Father's house the Father is watching,  and He says,  'All my sons are coming home.  My sons don't like pigpens because they do not have the nature of a pig.  They have the nature of a son.  They have MY nature,  and they won't be happy except in the Father's house.' "
 
Remember when the Prodigal son returned home to his father,  the pigs didn't go with him,  because they were happy in the pigpen.  But the son came to his senses,  realized he didn't belong there, and returned to his father.