Why do some people frequently say, "I'm not the perfect Christian"?

qualifier statements like that usually mean the person is about to say something they know they shouldn't. for example, many americans like to start sentences with "i'm not racist, but..." and then go on to say something blatantly racist, presuming that everyone around them then agrees to judge the statement as not being racist. taking the same thing into this xtian phrase (which i'm not really familiar with), maybe they think it works that way too. you know, "i'm not the perfect xtian, but i know so and so shouldn't go do blah blah.."