The Ramchal (Kitzur Kavanos) writes that during Neilah, every person can reach the level of Adam HaRishon before his sin.
The Mishnah Berurah writes, "One should be very diligent with Neilah because the pinnacle of Aseres Yemei Teshuvah is Yom Kippur, and the peak of Yom Kippur is Neilah. Everything follows the closing (הולך הכל החיתום אחר)." Neilah is when Hashem signs the decrees for the coming year, and therefore, we should daven this tefillah with a lot of kavanah.
The Gemara (Yoma 86) states that some aveiros are forgiven immediately. As soon as one does teshuvah, the aveirah is erased. Some aveiros need teshuvah and Yom Kippur to attain atonement. Some aveiros need teshuvah, Yom Kippur, and yesurim. But for the aveirah of chilul Hashem, he doesn't have kaparah until he dies. Nevertheless, the Meshech Chachmah explains that at Neilah, one can receive atonement for chillul Hashem, too.
The explanation is as follows:
The Yerushalmi explains that there is no forgiveness for chillul Hashem because no malach dares speak in the defense of a person who created a chillul Hashem. To suggest that the aveirah isn't so severe would in itself be a disgrace to Hashem's name. Therefore, when a chillul Hashem charge is brought to the heavenly court, all malachim remain silent, and no one advocates on that person's behalf. This is why there is no atonement, as long as the person is alive. However, at Neilah, Hashem alone judges us. The malachim aren't part of the process. When we plead before Him and beg forgiveness for the chillul Hashem we caused, He will forgive us.
The Ramchal writes that at Neilah, one can attain the level of Adam HaRishon before his sin. It is a time when we can become completely pure before Hashem. All sins can be removed. It is a time of immense compassion and forgiveness from Hashem.
The Rema (607:6) writes, quoting the Rambam, that Yom Kippur atones solely for those who believe in it. The Kotzker said that if people knew how pure their neshamos became on Yom Kippur, they would be cautious never to blemish their neshamos again.
The Rokeiach (217) writes, "Towards the end of Yom Kippur, Hakadosh Baruch signs the decree. There are malachim before his throne, some to the right side (saying that Hashem should sign for life), and some malachim are to the left (speaking against the Jewish nation), and all souls stand before His throne... Until the decree of the year is signed, each person can do teshuvah. If he breaks his heart with teshuvah, it will be accepted."
Trumas HaDeshen (278) writes, "The judgment isn't completed until Klal Yisrael finish their tefillos of Neilah. Even if it is already nighttime... nevertheless, the Jewish nation is beloved before Hashem, and His beis din follows after the order of their tefillos." Therefore, as long as we are still saying the tefillah of Neilah, it is still Yom Kippur, and we can still storm the heavens with our prayers and with teshuvah, and they will be accepted.
Every Moment of Yom Kippur
The Rambam writes, "Yom Kippur only atones for those who do teshuvah and believe in its atonement." Every moment of Yom Kippur can bring atonement for our aveiros (Krisus 7a). Therefore, Reb Shlomo Zalman Aurbach zt'l taught, a person shouldn't waste a moment of Yom Kippur. If a person wastes time on Yom Kippur, this means that he considers Yom Kippur to be the same as any other day, and that is a disgrace to Yom Kippur. Once, early Yom Kippur morning, Reb Shlomo Zalman awoke his grandson who slept in his room, and told him to recite Tehillim together with him. He said, "Each moment of Yom Kippur is more precious than gold."
The Last Arrow
A person was hunting in the forest, and before he knew it, it turned dark. He was terrified of the wild animals. Every sound sent panic into his heart, and he would shoot arrows in that direction, just in case it was a wild animal. Generally, it was just the rustling of the leaves or a small animal passing by. But then he saw a fierce lion approaching, and by now, he only had one arrow left. He knew that he must be careful and shoot the arrow accurately, on target, because there would be no second chance. This is how it is at the time of Neilah. We had the Aseres Yemei Teshuvah to improve our ways, but generally, we spent those days with unimportant deeds. Now is our last chance, and with a well-placed tefillah, we can be saved.
