C.S. Lewis (1898 –1963) was the chief forerunner of current Christian apologists.  His atheistic background was helpful for communicating with non-Christians since he understood what Christianity looks like from the outside.

Mere Christianity is one of his most popular books and is frequently quoted by modern day apologists.  It was originally presented as a series of broadcast talks during WWII.  This is a thorough book study of a resource that is just as valuable today as when it was first produced.