It is rare to find someone who truly and honestly believes that he is impervious to temptation. However, there could be certain situations where we may reasonably think that temptation can never enter our realm.
Repeated experience in specific situations may lead us to believe that we will not falter. We may think to ourselves that even if we are tempted we would never act upon it.
Are we really ever immune to temptation? Even if we cannot act on temptation is it really harmless?
The Talmud of the Land of Israel (Shabbath 1:3) tells of a pious Sage who taught that a person should not trust himself until he reaches old age. Only then does a person know himself well enough to know when he will or will not fail.
But everything changed one day when a spirit appeared before him in a tempting form. The Sage felt immense remorse for having such feelings. Then the spirit revealed itself and assured him that he was not actually oblivious to himself as his experience was not of natural causes.
It was at this moment that he realized that even though he knew himself, he still harbored inside him temptation that could break free if only the right circumstances were to come along.
The secondary lesson that the Talmud of Eretz Yisrael teaches us here is that feelings of temptation are themselves inherently reprehensible and not just because they can lead to transgression. The pious Sage felt remorse that he was capable of such feelings of temptation even though he did not act upon them.
May God grant us the wisdom and creativity to avoid situations of temptation even if when we can see no way that we could possibly falter!