The Congregation of Hashem
There are several instances where the Torah forbids certain individuals from entering קהל ה׳. Literally, this means that they may not enter the “congregation of G-d.”
The interpretation of this term, means that those who are listed in this category may not marry a Jewish girl of good lineage. This means that she is the daughter of a Kohein, Levite, or Israelite. They are permitted to marry a convert or a Mamzeret, a female who is the offspring of a forbidden relationship, such as the daughter of adulterers.
One category of those forbidden to enter the “congregation of Hashem,” and marry Jewish girls, refers to certain converts from specific. An Ammonite or Moabite male, who converts to Judaism, falls into this category. The same applies to a first and second generation convert from Edom (Eisav’s descendants) as well as first and second generation Egyptian convert.
Among born Jews, this prohibition also applies. The Mamzer, who is the offspring of an adulterous or incestuous relationship, is limited as to whom he is allowed to marry.
Another Jew that has these same limitations is one who is known as פצוע דכא or כרות שפכה. He has been mutilated or damaged in his private parts. If he was a Kohein, and an accident occurred after he was married, he and his wife may continue eating Teruma if they no longer have relations.
It is clear from these laws that the purity of the Jewish people must be maintained. There are different roles for the different levels of Jews. Those serving in the Temple, needed to be of the highest lineage. They had to be certain to marry the appropriate women to help them serve in their special role.
It is difficult to fully comprehend these laws, but we must trust that they come from the G-d that we are unable to comprehend. That should be enough for us.