The Past
Some people feel more “religious” by putting down other people’s religious observance. Instead of working to continue growing in their own avodat ha’kodesh (sacred work), they prefer bolstering their sense of religiosity by scorning or ridiculing the avodat ha’kodesh of others which they feel is deficient.
This practice reflects grievously poor middot (character traits), but also constitutes a violation of the prohibition of lashon ha’ra. It is absolutely forbidden to tell someone about the mistakes other people make in their religious observance. We should not be talking with people about other individuals’ level of observance in a derogatory way, whether it’s the observance of Shabbat, kashrut, or any other area of religious life. One is not allowed, for example, to say, “So-and-so knows nothing about the halachot of Shabbat. I was sitting with him last Shabbat and I watched how he ate his fish – he repeatedly violated the prohibition of borer (removing inedible entities from food).” Such talk is clearly forbidden. This applies regardless of what level of religious infraction is being reported – whether it is a Biblical transgression, the violation of a rabbinic law, or even the neglect of a humra (measure of stringency) which has been commonly accepted. All such reports reflect negatively upon the person being spoken about, and they are thus forbidden.
In fact, one is not even allowed to tell of his own past mistakes in his religious observance, even as part of casual, lighthearted conversation. It is forbidden to speak lashon ha’ra even about oneself, and thus, for example, one may not speak about how he used to work on Shabbat or eat non-kosher food, portraying himself in a negative light.
Certainly, it is strictly forbidden to remind somebody about the sins of his past. One may not, for example, ask a ba’al teshuvah about the non-kosher food he used to eat (such as, “You used to eat in Burger King? How did it taste?”). Recalling one’s sinful past constitutes lashon ha’ra and is strictly forbidden.
אין שכחה לפני כסא כבודך – G-d is capable of remembering everything forever. Nevertheless, G-d assures us that He will forget our misdeeds if we are prepared to forget the misdeeds committed by others, and never mention them.