And because the condition of your salvation was 'believing', not works, you can lose your salvation by not believing anymore.
I'm simply using the same logic that people use when they say that since salvation was not by works you can not lose it by not doing works.
See, people get into doctrinal trouble when they conclude that since salvation is not earned by works then salvation is by nothing at all, and so you can't lose it. But justification/salvation is not by nothing at all. It is through believing. You have to keep on believing to keep on being saved, just as you would have to keep on working to keep on being saved if salvation was by works.