If we're talking about evangelical Christians, the prevailing view is that they are sinful, but forgiven. They believe that they're not perfect, but that their imperfections have already been excused.

This gives us two points on which rationality fails to be as appealing. First, evangelical Christians don't have to doubt their understanding, they believe they know what it would look like if they were perfect, although they lack the fortitude to achieve it. Second, they have a forgiving entity to appeal to when they get things wrong.