Food Sacrificed to Idols

What are "things polluted by idols" (Acts 15:20)?
Food sacrificed to idols.

Who eats food sacrificed to idols today?
Billions of people around the world, including millions in Western countries.

What is an example of food sacrificed to idols eaten in the West?
Any food labeled "halal," which means it has been sacrificed to allah of Islam. To make halal meat, for example, the animal is made to face toward Mecca in Saudi Arabia, and its throat is slit after "bismillah allahu akbar!" which means "allah is greater!" is declared over it (Jesus is among those over whom Islam's allah is being declared greater).

Food sacrificed to idols is really sacrificed to whom?
To demons: "What am I saying then? That an idol is anything, or what is offered to idols is anything? Rather, that the things which the gentiles sacrifice they sacrifice to demons and not to God, and I do not want you to have fellowship with demons. You cannot drink the cup of the Lord and the cup of demons; you cannot partake of the Lord’s table and of the table of demons." (1 Corinthians 10:19-21)

Does the Bible tell us to ask if a food item has been sacrificed to idols?
No, but if that information is volunteered, to not eat it - Eat whatever is sold in the meat market, asking no questions for conscience’ sake; for “the earth is the Lord’s, and all its fullness.” If any of those who do not believe invites you to dinner, and you desire to go, eat whatever is set before you, asking no question for conscience’ sake. But if anyone says to you, “This was offered to idols,” do not eat it for the sake of the one who told you, and for conscience’ sake; for “the earth is the Lord’s, and all its fullness.” (1 Corinthians 10:25-28) - and halal label volunteers that information.

What was wrong with "things strangled and ... blood" (Acts 15:20)?
Blood was a pagan delicacy and strangled animals spilt blood when being cooked and/or eaten.

To recap, which four abstentions did the Jerusalem church instruct to the gentile church in Antioch?
Food that had been sacrificed to idols, sexual immorality, blood, and strangled animals that spilt blood when being cooked or eaten.

Were they being issued a new set of commandments or the most important commandments?
Peter had just rebuked the Pharisee believers, “Why do you test God by putting a yoke on the neck of the disciples which neither our fathers nor we were able to bear?" (Acts 15:10), and the most important commandments would not have been those four: But when the Pharisees heard that He had silenced the Sadducees, they gathered together. Then one of them, a lawyer, asked Him a question, testing Him, and saying, “Teacher, which is the great commandment in the law?” Jesus said to him, “ ‘You shall love the LORD your God with all your heart, with all your soul, and with all your mind.’ This is the first and great commandment. And the second is like it: ‘You shall love your neighbor as yourself.’ On these two commandments hang all the Law and the Prophets." (Matthew 22:34-40)

Then why did the Jerusalem church instruct those four abstentions?
The common thread is paganism. Blood was a pagan delicacy. Animals were sacrificed to idols at pagan temples back then. And prostitution at pagan temples was an especially popular form of sexual immorality. Prior to this incident, the young gentile church had been caught between the Jewish church on the one side and paganism on its other side. Negating the need for circumcision eliminated the barrier between the gentile and Jewish churches, while the above four abstentions strengthened the barrier between the gentile church and paganism, from which most of its members had come. Satan had created a conflict to try to split the gentile church from the Jewish church, but God used the conflict to unify them.

In context, what does "Moses" (Acts 15:21) mean?
The myriad laws for Jews that were derived supposedly from the Ten Commandments that Moses received from God.

So what is James saying to the Jews in the Jerusalem church?
'Look, we had all these laws preached and "read in the synagogue every Sabbath" (Acts 15:21) and we couldn't keep them, so "we should not trouble those from among the gentiles who are turning to God" (Acts 15:19) by imposing more commands beyond these four.'