It is easy to justify a small untruth. It is small and insignificant and has little impact one way or another. Except: With that small lie, I have moved the boundary of my moral compass ever so slightly, but it has moved which makes it easier next time to move it just so much more until the boundary become malleable and subject only to what I can justify.
This last statement is very important because most of us will not tell an untruth that we cannot justify. That is why lies start small. It is something that is not too hard to justify and if the boundary of truth moves ever so slowly we can keep up with it. And the thing about self justification is that because a lie creates dissonance between our belief system and our actions it becomes necessary for us to justify the action and pretty soon we actually believe our own justification, skewed as it is.
I have met very bright, sometimes brilliant Christian leaders who have caused immeasurable hurt to others through promises not kept, untruths, financial mismanagement, unjust firings and the like. And they have a justification for all of it. Nothing is their fault, they don't take any responsibility and their justifications while absurd to others make perfect sense to them. They believe their own lies. Why? Over time their moral boundaries moved ever so slightly until they were now a great distance from where they started and the only one who does not see it is the individual himself or herself.
The problem with small untruths is that multiplied over and over they become large untruths. And one day, there is no way to even confront those lies because the justifications are believed by the one who has moved their moral boundaries. And it happens to believers and non-believers alike. Even Christian leaders!