The Literal Commandment
No Ammonite or Moabite shall enter the assembly of the LORD; none of their descendants, even to the tenth generation, shall ever enter the assembly of the LORD, because they did not meet you with food and water on the way when you came out of Egypt, and because they hired against you Balaam the son of Beor from Pethor of Mesopotamia, to curse you." (Deuteronomy 23:3-4)
The literal commandment is that no Ammonite or Moabite shall enter the assembly of the LORD down to the tenth generation.
Messiah Says
Messiah implicitly affirmed this commandment when He spoke about the Law:
"Do not think that I came to abolish the Law or the Prophets; I did not come to abolish but to fulfill. For truly I say to you, until heaven and earth pass away, not the smallest letter or stroke shall pass from the Law until all is accomplished. Whoever then annuls one of the least of these commandments, and teaches others to do the same, shall be called least in the kingdom of heaven; but whoever keeps and teaches them, he shall be called great in the kingdom of heaven. (Matthew 5:17-19)
Pictures of Messiah
Ammon (Deuteronomy 2 & 3) and Moab (Numbers 22-24) sought to curse Israel when they left Egypt. Because of their active animosity towards Israel, they are not allowed to participate in the blessings G-d bestows upon Israel.
In a similar fashion, those who are enemies of G-d (all of us before we come to faith! Romans 5:10) cannot enter into the blessings that come from being a part of the bride of Messiah.
How Messiah Fulfilled
Messiah never fulfilled this commandment because He never physically married a woman (Moabite, Ammonite, or otherwise) nor was He in a position where He would have been required to prevent such a union between a Jew and a Moabite or Ammonite.
Traditional Observance
The traditional observance of this commandment is that Jews are forbidden from marrying Ammonite and Moabites (or their modern-day equivalent). They are allowed to marry converts to Judaism.
There is precedent in Scripture for such conversion in the story of Ruth (who was from Moab). She declares to Naomi, "where you go, I will go, and where you lodge, I will lodge. Your people shall be my people, and your God, my God." This can be seen as a declaration of her intent to convert to Judaism and become a member of the covenant.
Other Notes
We are able to fulfill this commandment today and we should not marry Ammonites or Moabites who are not part of the covenant.
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