A vascular surgeon I know went from being an agnostic-atheist to a staunch Christian reading Lee Strobel's books: The Case for Faith and The Case for Christ.

The first one made him realize who was he kidding, of course there is a God.
The second one "scared me to death."

He is much more effective in witnessing than I am, because having been a nonbeliever to just a year or two ago, he knows where they are coming from, and above all, has the patience to sit a listen to what they have to say.  He is very skilled at making them realize for themselves the consequences of their belief, much like Michael Medved is so good at debating liberal elites, asking them question after question.