Until Peter's vision (Acts 10 (NKJV)),
in which he is told that Christianity is intended for all mankind, "… God has shown me that I should not call any man common or unclean",
Christianity was entirely Jewish, the only difference being the belief that Jesus was the promised Messiah.
Christians and Jews lived, worked, worshiped, and studied together.
But now Gentiles were wanting to become Christians, and there was no obvious path for them to follow.
Consider how the Gentiles that were considering converting were learning Christianity:
So when the Jews went out of the synagogue, the Gentiles begged that these words might be preached to them the next Sabbath.
— Acts 13 (NKJV)
In Antioch, Paul was teaching at an existing synagogue on the Sabbath.
In other places, where there was no Paul or other teacher continually available, interested Gentiles were expected to attend regular sabbath services at their local synagogue.
There, they could learn the basic truths about God, creation, and the history of Israel.
During the other six days, they could study the scriptures for themselves.
Until recently though, books of all kinds were rare and expensive.
Individual people did not own Bibles, and there were no lending libraries; anyone wanting to read the scriptures had to go to a synagogue:
[The Bereans] were more fair-minded than those in Thessalonica, in that they received the word with all readiness, and searched the Scriptures daily to find out whether these things were so.
Therefore many of them believed, and also not a few of the Greeks, prominent women as well as men.
— Acts 17:11–12 (NKJV)
But, even though Jews welcome Gentiles to their services, they require that those people be seriously interested, not simply curious.
Giving idol worshippers, blasphemers, thieves, etc. access to the holy documents would profane them.
Modern Judaism has a concept of universal laws that apply to all humanity, not only to Jews:
The Seven Laws of Noah (Hebrew: שבע מצוות בני נח<200e> Sheva Mitzvot B'nei Noach), … are a set of imperatives which, according to the Talmud, were given by God as a binding set of laws for the "children of Noah" – that is, all of humanity.
According to Jewish tradition, non-Jews who adhere to these laws are said to be followers of Noahidism and regarded as righteous gentiles, who are assured of a place in Olam Haba (עולם הבא<200e>, the world to come), the final reward of the righteous.
The Seven Laws of Noah include prohibitions against worshipping idols, cursing God, murder, adultery and sexual immorality, theft, eating flesh torn from a living animal, as well as the obligation to establish courts of justice.
— Seven Laws of Noah - Wikipedia
The meat prohibition is derived from Genesis 9:4 (NKJV), "But you shall not eat flesh with its life, that is, its blood".
Ignoring the four laws that almost all religions would consider obvious (blasphemy, murder, theft, injustice), and which the proselytes would already be following, leaves these three:
- Worshipping idols.
- Sexual immorality.
- Eating meat that has not bled to death (e.g. strangled or diseased, or still alive (oysters)).
These prohibitions correspond almost exactly to the list defined by the Council of Jerusalem.
The controversy in Acts 15 was over exactly what potential converts to Christianity (and implicitly to much of Judaism) should be required to do before their conversion.
Some Christian leaders felt that these Gentiles should first convert to Judaism.
They would have to be circumcised and follow all relevant laws before they could become Christians.
Others realized that that would be an impossible requirement: no one would want such an operation before they were even allowed to learn about the reason for it.
They also knew that Christianity was based on a new covenant with God, one available to all mankind, and that conditions specific to the old covenant were meant for the descendants of Abraham, not for all of humanity:
And God said to Abraham: “As for you, you shall keep My covenant, you and your descendants after you throughout their generations.
This is My covenant which you shall keep, between Me and you and your descendants after you: Every male child among you shall be circumcised;
and you shall be circumcised in the flesh of your foreskins, and it shall be a sign of the covenant between Me and you.
— Genesis 17:9–11 (NKJV)
The final decision was that the Christians shouldn't require any signs of sincerity beyond what the Jews already required of Gentiles: they should refrain from activities forbidden to all of Noah's descendants: temple prostitutes, meat sacrificed to idols, meat containing blood.
Notice what James concluded:
“Therefore I judge that we should not trouble those from among the Gentiles who are turning to God,
but that we write to them to abstain from things polluted by idols, from sexual immorality, from things strangled, and from blood.
For Moses has had throughout many generations those who preach him in every city, being read in the synagogues every Sabbath.”
— Acts 15 (NKJV)
- Requiring circumcision of potential converts would be counter productive; rather than encouraging them to learn the truth it would scare them away.
- Requiring only what the Jews already expected for admission to their synagogues would be sufficient.
- Each week, the potential converts could then attend services and study the foundations of the religion.
Despite how most people misinterpret this ruling, it had nothing to do with biblical dietary laws, nor anything to do with Christianity versus Judaism.
Was there some implicit set of laws that was not referenced, but would be understood by both parties to be in effect?
Yes, effectively everything. This ruling was not about what rules Christians should or should not follow, but about what potential Gentile converts needed to do to be allowed to learn in Jewish synagogues.