City of Fire by Grace Livingston Hill

Home from college, Lynn is heartbroken to find her childhood friend Mark, has fallen away from his faith in God. After making some poor life choices, Mark is caught up in a local scandal. Billy, one of the bright young boys from Lynn's Sunday School class, is determined to clear Mark's name. But at what cost? And will Billy's betrayal hurt the people he's trying to protect? - Summary by Emily Grace