Salvation's Currency

When shopping at Piggly Wiggly, Publix, Wal-Mart, or Winn-Dixie, most of us pay with credit cards. Of course, whatever we charge on those cards has to be paid back. Believe it or not, there is a freely given, exchangeable credit that can be used to secure our ticket to heaven. Just what exactly is that supernatural, non-monetary medium of exchange, one might ask?
Well, it is not riches. It is not doing good works. It is not doing good deeds or even living an exemplary life—not that those are in any way negative things. It's just that those attributes or traits do not, nor will not, purchase the key that opens the gates to heaven. So what is this wonderful, redeemable 'currency' we can gain?