Comment 8 by Steve Zara :

If I may re-use Richard's analogy, there can be a gentle path down "Mount Unbelievable" towards lesser levels of belief, not a sudden plummet into the (supposed) abyss of atheism.

This is definitely the right approach, unless as mikenlynn noted, they're already tilting towards atheism.

One of the arguments that I use actually gets close to getting emotional reactions from religious people. I talk about accomplishments of the human intellect such as abolishing slavery, declaring equality of all people, rights to liberty, life, pursuit of happiness etc.

Everyone -especially in the western world- loves talking about them since the enlightenment era took place in the west. What most people don't know however is that these movements were in conflict with scripture (which condones owning slaves, stoning women etc).

Try it, often times I hear people disbelievingly say, "no way... are you saying it says this stuff is in my Bible?" I have found this as a successful way to make people take the first step towards discussing religion skeptically.