Why is Rosh Hashana considered the Day of Judgment and what exactly is being judged and how?
Rosh Hashana was ordained as a day of judgment for two reasons: The first is that on this day, the first day of the Hebrew month of Tishrei, the creation of the word was completed (Rosh Hashana 10b) and it was the Divine intention that the world be ruled by the trait of strict justice (see Rashi Gen. 1:1 from B.R. 12:15). Hence the beginning of the year was marked as the Day of Judgment. The second reason is that on this day Adam was judged, he repented, and he was forgiven (Lev. Raba 29:1).
Rambam (Teshuva 3:1-3) teaches that every person has both merits and transgressions. If one’s merits outweigh one’s transgressions, he or she is considered to be righteous and judged for life and blessing. If the transgressions outweigh the merits, one is considered wicked and judged negatively, as in the verse “I have wounded you…because of the enormity of your sins” (Jer. 30:14). If they are basically equivalent, the judgment hangs in the balance till a person tips the scale in either direction affecting the judgment accordingly.
The same is true regarding an entire country. If the collective merit of all the inhabitants is greater than their sins, the people are considered to be righteous and the country is judged favorably for the coming year. If their collective sins are greater than their merits, they are considered to be collectively wicked and the country is judged for damage and destruction, as the verse states, “For the outcry from Sodom and Amorah is great” (Gen. 18:20). The same is also true of the entire world, as in the verse regarding the Flood, “And G‑d saw that man’s evil was great” (Gen. Gen 6:5).
However, this judgment is not quantitative but rather qualitative. There are individual acts of merit that far outweigh many sins, as in the verse, “Because in him there is found some good” (Kings 1 14:13). And likewise, there are individual sins that outweigh many acts of merit, as in the verse, “One sinner can cause much good to be lost” (Eccl. 9:18). The determination of this is dependent solely on the judgment of G‑d, whose knowledge is all-encompassing, for only He can truly evaluate merit and sin.
Therefore, each person should see himself during the entire year as if he were half meritorious in order to be encouraged by the recognition of his goodness, and half guilty in order to be spurred to rectify what’s wrong or missing. Likewise, he should consider his country and even the entire world as being in the same state of hanging in the balance. Thus, if he commits one single sin, he is capable of tipping the scale of transgression for himself, his immediate environment, and the entire world toward destruction. Similarly, if he performs one mitzva, he can tip the scale of merit for himself, others and the world causing salvation and deliverance. This is as in the verse, “And the tzadik is the foundation of the world” (Prov. 10:25). Meaning, because he is a righteous tzadik, he tips the scale of the world to the side of merit and saves it.