Our Gemara on Amud Beis discusses the idea that a person is considered eligible to be counted as part of the Paschal sacrifice once he completes the rituals of purification, even though he will not become fully purified until after the Sun sets. The Gemara employs a formulation known as, “Shimsha Memeyla Arev - The Sun sets by itself”, i.e. the person has completed all the activities he must do, and since the Sun will set of its own accord, he is granted a presumptive eligibility of the sacrifice that allows him to be part of the assigned group. (A Paschal sacrifice requires that it to be slaughtered with a specific group of people who have committed to partake of that sacrifice. One cannot join a sacrificial Passover meal after the animal was slaughtered. See Mishna Zevachim 5:8).
Although the simple reading of the Gemara implies this, Rambam (Laws of Pesach Sacrifice 6:2) limits this special dispensation to impurities that the do not cause the Nazir to restart counting the days of his Nazirhood. Thus, a person in a typical case of exposure to a corpse would not be able to participate in the Paschal sacrifice, even if he completes his 7th day of purification on the 14th of NIssan when the Paschal sacrifice is slaughtered. Despite the fact that he will become eligible to eat sacrifices once the Sun sets, he was still ineligible at the time of the slaughtering. Ra’avad follows the straightforward reading of our Gemara and thus allows the person who completed his purification rituals on Erev Pesach to eat from the Pesach on the evening, after the Sun sets, because he was in his eligible to become fully purified by the time the evening approached.
Bnei Yissaschar (Nissan 4) explains the dispute between Rambam and Ra’avad as representing symbolic truths about repentance, which are enacted in the Paschal sacrifice, rules and performance. He explains that there ar two aspects to repentance: The personal resolve and internal change of orientation, and then the actual cleansing and full restoration of the spiritual damage. When a person resolves to behave righteously and renounce his sin, at that moment he is now a Tzaddik — in terms of how he is defined. Yet, the lingering spiritual damage and impurities require a transformation in behavior and structure of self which only can happen over time, This is why there are types of sins where the tradition tells us that even after repentance it can take until certain afflictions are experienced, or Yom Kippur, or even death to fully expiate the sin (Yoma 85b). But that is not to say he hasn’t immediately achieved the status of a Tzaddik, just he still has lingering toxicities that take time and lived experience to purge.
Symbolically, the immediate resolve to repent is represented by the immediate purification, which is achieved to the extent that he is no longer defiled and can even eat Terumah. However, the final status which represents full spiritual recovery and allows him to eat from sacrifices requires the setting of the Sun. This represents the physical change of behavior and removal of all toxicities which requires time and process.
The Jews in Egypt had to make a bold step and renounce their idolatrous practices by preparing a lamb for the Paschal offering. Even though the idolatry left all lingering spiritual stains, according to the Ra’avad, God was granting special provisional eligibility and acceptance regardless. This is symbolically represented by allowing the Jews to eat from the Paschal sacrifice because they WILL complete their purification when the Sun sets.
But what about the Rambam’s position? Understand this, we must consider his unique formulation in Laws of Repentance (2:2):
וּמַה הִיא הַתְּשׁוּבָה. הוּא שֶׁיַּעֲזֹב הַחוֹטֵא חֶטְאוֹ וִיסִירוֹ מִמַּחֲשַׁבְתּוֹ וְיִגְמֹר בְּלִבּוֹ שֶׁלֹּא יַעֲשֵׂהוּ עוֹד שֶׁנֶּאֱמַר (ישעיה נה ז) "יַעֲזֹב רָשָׁע דַּרְכּוֹ" וְגוֹ'. וְכֵן יִתְנַחֵם עַל שֶׁעָבַר שֶׁנֶּאֱמַר (ירמיה לא יט) "כִּי אַחֲרֵי שׁוּבִי נִחַמְתִּי". וְיָעִיד עָלָיו יוֹדֵעַ תַּעֲלוּמוֹת שֶׁלֹּא יָשׁוּב לְזֶה הַחֵטְא לְעוֹלָם שֶׁנֶּאֱמַר (הושע יד ד) "וְלֹא נֹאמַר עוֹד אֱלֹהֵינוּ לְמַעֲשֵׂה יָדֵינוּ" וְגוֹ'. וְצָרִיךְ לְהִתְוַדּוֹת בִּשְׂפָתָיו וְלוֹמַר עִנְיָנוֹת אֵלּוּ שֶׁגָּמַר בְּלִבּוֹ:
What constitutes Teshuvah? That a sinner should abandon his sins and remove them from his thoughts, resolving in his heart, never to commit them again as [Isaiah 55:7] states "May the wicked abandon his ways...." Similarly, he must regret the past as [Jeremiah 31:18] states: "After I returned, I regretted."[He must reach the level where] He who knows the hidden will testify concerning him that he will never return to this sin again as [Hoshea 14:4] states: "We will no longer say to the work of our hands: `You are our gods.'"He must verbally confess and state these matters which he resolved in his heart.
Much ink has been spilled over the ages to fully articulate what the Rambam intended, but plus or minus, it seems to be along the lines that if even God can testify to his sincerity, this proves deep resolve. Of course, most of the time, people will return to sin. (We even ask for forgiveness in Shemoneh Esre of Maariv right after Yom Kippur, minutes after we turned over a new leaf!) Nevertheless, at that moment of repentance, his sincerity was so thorough that even God endorses it. Benei Yissaschar says that this represents the part of repentance that can only be achieved through God’s help. Thus, the person goes as far as he can, but then God does the rest. So too, according to the Rambam, his understanding of the legalities of the Paschal sacrifice support this symbolic message. Repentance alone cannot work without God’s assistance. The setting of the Sun, which is out of human control and in God’s hands, represents the final stage of repentance that only God can do. The Paschal sacrifice, which was the Jewish people’s renouncing of idolatry, while an important human step, still needed God’s final step to become purified.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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