One-fifth of all outsiders, regardless of age, admitted they 'have had a bad experience in a church or with a Christian that gave them a negative image of Jesus Christ.' This represents nearly fifty million adult residents of this country - including about nine million young outsiders - who admit they have significant emotional or spiritual baggage from past experiences with so-called Christ followers. Among pastors of Protestant churches, three-quarters said they often encounter people whose negative experiences create major barriers to their openness to Jesus. (p. 31, emphasis added)
These figures come from Unchristian, a recent book by the highly respected Barna Group, an evangelical polling firm that has done work for SONY, Walden Media, Easter Seals, CARE, the ONE Campaign, the Humane Society, the Gates Foundation and NBC Universal.
A summary of their findings can be found here. Go here for a 4 min, 30 sec interview with one of the authors.