Caution refers to being cautious and protecting one’s self from all iniquity or any possibility of sin — even more than one is careful to avoid fire or a sharp sword. For going against the will of Hashem and to cause sorrow, anger and blemish in the Worlds Above is more bitter than a thousand deaths and the punishment of Gehinnom. It is worse than all the calamities of the world. Therefore, a God-fearing Jew will “run past one hundred gates of that which is permitted in order to avoid even one that which is prohibited” (cf. Chavas Halevavos, Sha’ar HaTeshuvah, ch. 5). He will not listen to the voice of his Evil Inclination who makes light of prohibitions and provides all sorts of false arguments to allow them; every day the Inclination makes the person grow accustomed to sins, until they feel completely permissible.
One who is yet alive will take to heart the thought that even the slightest sin causes the greatest harm. It shows that he does not care about the honor of his Creator since he is not putting in the effort to fulfill His Will. So, a person who fears God should flee and distance himself from something that is only doubtfully forbidden or that is a matter of halachic dispute; even if ninety-nine authorities allow it but one says that it is forbidden — he should be concerned for that opinion that forbids it [unless that opinion has been rejected by the codifiers completely].
To what can this be compared? To something that ninety-nine doctors say is safe and only one says that it is unsafe and can be life-threatening if ingested. What fool will not take that lone opinion into consideration? Obviously, any intelligent person would say to himself, “What do I need this problem for? Why should I risk this danger? I am better off playing it safe!” Now, if a person is this cautious with regard to the temporary life of this World, he should certainly be concerned about the life of the World to Come and about possibly causing sorrow and anger before the Holy One, blessed is He. These types of thoughts are beneficial to saving a person from all sin and it leads him to piety. This is the straight path of those who fear Hashem and think about His Name. One who walks with innocence — his God will be with him.