[FNG]

OK, first, karma is a Hindu doctrine that's not compatible with Christianity. What it means, in short, is that your actions now create your future environment. e.g., if you hurt people, they're more likely to be hurtful in the future, i.e. this lifetime or some future lifetime (reincarnation is a given, obv). The more you do this, the more likely you are to run into someone you have hurt (or they have hurt, or so on).

I could see how an atheist who believed in reincarnation (if you could find one) could believe in karma, but not how a theist who didn't could. One lifetime is too small a sample size to expect the odds to approach expectation.