You could start with geography. Point out how similar beliefs have been localized to specific regions until relatively recently - and that the change is due to improvements in travel technology more than any changes in beliefs of any individuals or groups. In other words most theists follow some version of the predominant religion of the region in which they grew up. By the looks of things Christians think God only likes Americans - but that's not what the bible says. Actually, the bible doesn't show god liking very many people at all but that won't get you far in the beginning.
Point out the atrocities in the Bible, and ask if those things match up with the God they believe in. Cast doubt on the divine inspiration and point out the human fingerprints all over the manuscript. Talk about how it's been used to justify horrible acts, and again, stress the human behavior that would lead to creative translations of the word of god.
Don't let anyone off with "the NT changes everything" excuses either - there are several threads in here that explain in great detail why Christians can not dismiss the OT due to Jesus - if they try, it invalidates everything their religion is based on and really drives a spike into the acceptance of Jesus as the predicted savior.
Buy everyone a copy of The Brick Bible, both OT and NT for Christmas this year. It wasn't deliberate on my part, but I contributed to someone's emerging deconversion that way this summer.
You could also chip away at the schisms between denominations, calling into question some of the minutia that divides various sects and the lengths people have one to to preserve them with no discernible impact to any of their lives - no punishment from God for following the "wrong" version, so how can anyone tell which one is the "right" one? All denominations of christianity claim to get their instruction from the Bible, so how can so many different interpretations exist and ALL be coming from the same book dedicated to the teachings of the same god? Why wouldn't God want to clear up the confusion by making it clear who is correct? That points to an indifferent god by the most generous terms, and not what (they think) the bible says is true. It's certainly not what they are being told in church.
My issue is not with god either. It's entirely with what people are doing in service to their beliefs in such a being that pulled me into vocal and active opposition. Even the bible talks about keeping your practices private - you could use that as well to point out how wrong evangelical behavior is.