Acts 15:21 simply means there are Jews in every city who have been brought up to know the law of Moses, and some of these Jews have become Christians and have joined themselves with the Gentile believers; and the Gentile believers are urged to behave in such a way so they do not become a stumbling block to any of these Jews while in their company in the fellowship, or other Jews whom they need to seek to win for Christ.
To understand 15:21 it is necessary to understand the whole passage:
Gentiles and Jews are becoming believers and joining together in one body, the Church. The Jews have been brought up on the law of Moses and "are zealous for the law". Gentiles cannot see why the law should remain in force seeing they are free in Christ. Their consciences are differently instructed. So as not to be a stumbling block to the Jewish believers (and those Jews interested in the Gospel) the Gentile believers are asked to do four things as in Acts 15:20 and 29.
The four things in the list appear to be very arbitrary: to abstain from eating things offered to idols, from blood, from things strangled, and to abstain from fornication. Why prohibit fornication which is a subcommand of the command "Thou shalt not commit adultery" but not any of the other ten commandments?
For instance, why does the list not include "Don't steal" and "Don't murder"? The answer is found in Leviticus chapters 17 and 18. These chapters deal with those small number of Mosaic regulations which are binding not just on the Israelites but are also binding on those "strangers"/foreigners/gentiles who have decided to make their home in the Promised Land amongst the Israelites.
These gentiles are not bound by any other ceremonial laws, such as circumcision, nor do they need to attend any of the Jewish festivals. They do not need to keep the whole ceremonial law.
As for the moral law, as summarised in the ten commandments, of course gentiles must seek to keep it, and that is taken for granted by the Jerusalem Council in this recommendation. The moral law is not the issue here in Acts 15.
In the King James Version "stranger" or "strangers" is found in Leviticus 17 verses 8, 10, 12, 13, 15, and in Lev 18 verse 26. This is the passage that the list of four recommendations in Acts 15:29 is referring back to, even in the same order:
- They should not eat things offered to idols, Lev 17:1-9;
- They should avoid eating blood, Lev 17:10-14;
- They should avoid eating things strangled or dying in an "unapproved" way, Lev 17:15-16 (- it seems that, amongst the gentiles in NT times, strangulation was a way of killing animals that were going to be eaten);
- They should not commit fornication, Lev 18:6-26.
It follows from this that the recommendation to abstain from "fornication" in Acts 15 is an appeal to abide by that definition of fornication as found in Leviticus chapter 18, a definition that the Gentiles might have needed instruction in.
These recommendations are not recommendations to which gentiles are to be bound to at all times or forever: they only are given "because Moses has had throughout many generations those who preach him in every city, being read every Sabbath in the synagogues", the recommendations are given so as to keep the fellowships harmonious which have both Jews and Gentiles, and to help gentiles to evangelise Jews in their daily lives. When Jews are not present Gentiles are not bound by these recommendations.
For instance, if you wanted to invite a Muslim couple who are neighbours round to have a meal, in the hope of sharing the Gospel with them, I think you would not cook up roast pork and tell them their rules against pork are silly and don't apply any more. They will be offended and you will have lost all opportunity for sharing the Gospel with them forever. But you can eat pork within the privacy of your own family and when you invite non-Muslims or non-Jews for a meal. You can eat things offered to idols, and blood, and things strangled, when Jews are not present: we are told this, for example see 1 Corinthians Chapter 8. Do not use your freedom in the Gospel from the Mosaic law to cause others to stumble.
Bible passages relevant to this passage, then, include Leviticus chapters 17 and 18; Romans 14:1-23; 1 Corinthians 8:1-13; 1 Corinth 9:19-23; 1 Corinth 10:23-11:1; Ephesians 2:11-22; Colossians 2:8-23; Mark 7:5-23.
Romans 14 is especially interesting because Romans is not written to correct abuses as some of the other letters; Romans is a general declaration of the Gospel and then a declaration of the behaviour that God desires as a consequence of faith in Christ: and a whole chapter is dedicated to the issue of not causing offence to other believers in areas where your conscience is differently informed to theirs.
See also Acts 16:3 where Paul circumcises Timothy because his mother was Jewish so he should have been circumised already according to the law. The reason given for circumcising Timothy is not because the law still needs to be kept but "because of the Jews", i.e. for the same reason, so as not to cause unnecessary offence, and because the Jews still think the law needs to be kept. Paul and Timothy are free, they don't have to keep the law; but they are also free in the sense that they can keep the law for the benefit of the consciences of others:
"For though I am free from all, I have made myself a servant to all, that I might win the more; and to the Jews I became as a Jew, that I might win Jews; to those who are under the law, (I became) as under the law, that I might win those who are under the law... I have become all things to all men, that I might by all means save some. Now this I do for the gospel's sake, that I may be partaker of it with you." 1 Corinthians 9:19-23.
So Acts 15:21 does not mean that new Gentile believers will go to the synagogue. Why would they? New Christian fellowships are being established everywhere and believers, once they understood, would leave the synagogues and never see them as a place to worship God. It was from the synagogues that a large measure of early opposition to the Gospel originated. Neither does it mean that new Jewish believers would stay in the synagogue.
The same interpretation was given by Charles Taze Russell founder of the group which became known as the Jehovah's Witnesses:-
“The answer ignored every feature of that law, except four points; and the first three of these were mentioned no doubt as a basis of common fellowship between those who had been Jews and those who had been Gentiles, namely, (1) abstaining from meats that had been offered in sacrifice to idols; (2) abstaining from animal food that had not been killed after the manner of the Jews; (3) abstaining from the eating of blood. It would be almost impossible for those who had been reared as Jews to ignore these three points, and if the converts from the Gentiles did not observe them it would be a constant barrier to their social intercourse.” The Watchtower 05/15/1897, p. 153 (Reprints p. 2158)
If Charles T. Russell went into a Kingdom Hall today and gave his explanation of Acts 15:19-21 he would be told he was proud and puffed up and not willing to submit to the Discreet Slave class, which gives the food appointed to eat at the proper time. And if he persisted he would be disfellowshipped.
Christians who are working amongst Jews today need to think what steps they should take in order to minimise offence amongst the Jews they are seeking to win for Christ: a consideration of the four recommendations of Acts 15:20 might be a good place to start.
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How do Protestants counteract the Witnesses stance on Blood Transfusions?