The hidden (sins) are for Hashem, but the revealed (sins) are for us and our children forever, to carry out all the words of this Torah (Devarim, 29:28). This climax of two lengthy descriptions of the curse which will befall Am Yisroel if it strays from Torah observance delineates the proper response of the observant community to individual sinners.
Hashem does not punish Am Yisroel for the hidden thoughts or sins of individuals. However, we are required to destroy the evil of revealed sins from our midst. Otherwise, the entire community will be punished. This applies only after we became responsible for one another at Har Grizim and Har Eval (Rashi).
The sense of community, which began in earnest when we crossed the Yarden into Eretz Yisroel, demands a response to deviation from communal Torah norms. Beis din must punish violations of Torah law in the biblically prescribed manner of death or flogging.
At Har Eval, the entire people gathered and cursed twelve types of sinners, whose sins are typically committed in private. As a result, Hashem will punish the sinners, thereby sparing the community at large (Rashbam 27:15).
Failure to punish public sinners, or to curse private sinners, is construed as acceptance of these sins as a legitimate alternative to strict adherance to Torah.
This can cause widespread non-observance. Hashem punishes the people so that they restore communal religious discipline.
Nowadays, beis din no longer imposes corporal punishment. Even Talmudic extra-legal measures to eliminate grievous offenders ("moridim") no longer apply. In Talmudic times, people realized that such offenders threatened the well-being of the community, and that their elimination was beneficial. Nowadays, such actions would be viewed as strong-armed thuggary and would be counterproductive (Chazon Ish Y.D. 2:16).
Similarly, cursing wrongdoers would backfire in today's world, when even the more recently practiced cherem has fallen into disuse. In an open society, these curses would not deter potential sinners and would only yield more hatred within our people. Moreover, the majority of those who violate Halacha do so out of ignorance, and are not included in the biblical curses (Ramban 29:28).
How, then, can we maintain our own limited community in a postmodern world which denies absolute truth, at a time when punishing and cursing wrongdoers are not viable options?
First, we must make Torah life exciting and attractive. We can no longer take observance for granted in an open global marketplace of competing ideals and values.
Second, we must adapt the biblical precedent of proclaiming sinful activity as unacceptable. While we can no longer punish or curse offenders, we can, and must, excoriate sinful activity. We must distinguish between the person and the action.
The primary purpose of proclaiming sinful behavior and heretical doctrine as such is the preservation and strengthening of the existing Torah community. The formulation of timeless principles in a trendy world has the additional benefit of proclaiming the essence of Torah Judaism before the entire world. After all, all of the nations (29:23, 24) talk about us and our covenant with Hashem.
Finally, a clear and unapologetic articulation of the Torah's boundaries and its intolerance of deviationist practices projects the image of a serious religious commitment to Jews who are searching for true meaning. "The hidden for Hashem" refers to assimilated Jews as well (Rashi Tehilim 87:6), and we do His work by teaching uncompromising eternal law, even as we show appropriate compassion and understanding for the "tinokos shenishbu" whose kiruv we crave (Rambam Hil. Mamrim 2:2).
Hanistaros - evil thoughts (Rashi) and deeds (Rashbam) hidden from the public, sins that are unknown to those who commit them (Rambam), and assimilated Jews - lashem Elokeinu. Let us learn and fulfill our responsibilities for nistaros and niglos - to carry out all the words of the Torah.