Catholic Ragemonkey

Am I missing something here?
I really am confused and I would appreciate it if any reader could shed some light on the following situation. As I was returning from anointing someone at the hospital this morning, I read the following message on the sign board of a local church:

The acid test of a Christian is forgiveness.

I don't get the "acid test" part. Do you think the person responsible for the sign really intended "litmus test" or is there some other meaning to "acid test" I am missing?