Why universalise what God has particularised?

If we we were never included in the Old Covenant why are we so concerned about being in the Old Covenant? This is further thinking from that book I’m reading. We are in the New Covenant - in Christ!

Paul, writing to the Galatians, says: “Now before faith came, we were imprisoned and guarded under the law until faith would be revealed. Therefore the law was our disciplinarian until Christ came, so that we might be justified by faith. But now that faith has come, we are no longer subject to a disciplinarian, for in Christ Jesus you are all children of God through faith. As many of you as were baptised into Christ have clothed yourselves with Christ. There is no longer Jew or Greek, there is no longer slave or free, there is no longer male and female; for all of you are one in Christ Jesus. And if you belong to Christ, then you are Abraham’s offspring, heirs according to the promise”. (Gal.3: 24-29).

When he says “we were imprisoned and guarded under the law”, he is talking about the Jews. The Gentiles were never under the law.

We see this in the Old Testament itself:

“No one whose testicles are crushed or whose penis is cut off shall be admitted to the assembly of the Lord.

2 Those born of an illicit union shall not be admitted to the assembly of the Lord. Even to the tenth generation, none of their descendants shall be admitted to the assembly of the Lord.

3 No Ammonite or Moabite shall be admitted to the assembly of the Lord. Even to the tenth generation, none of their descendants shall be admitted to the assembly of the Lord”. (Deut.23: 1-3)

And as far as other peoples were concerned:

“When you approach the frontier of the Ammonites, do not harass them or engage them in battle, for I will not give the land of the Ammonites to you as a possession, because I have given it to the descendants of Lot.” (Deut.2: 19).

None of the people listed above were included in the Old Covenant, so they were not expected to live by it.

When I first discovered the Bible at the age of 9 I read it from the beginning and when I reached Deuteronomy 23: 2 (in the KJV) I was horrified. I took it to mean that God didn’t want me - down to the 10th generation. But it doesn’t say that. It says that I’m not included in the Old Covenant.

So, with all the above exemptions, why did we ever assume that the Old Testament laws applied to everyone? They didn’t apply in Old Testament times, so why did we impose them on every Gentile nation that the Church went to in the days of the New Covenant?

Why did we make a covenant that God made with Israel into universal laws and then condemn people for not being in the covenant from which God had excluded them?