In Acts 15 the apostles and elders met at Jerusalem to resolve a dispute over whether Gentile converts were required to be circumcised.  They concluded that circumcision should not be required of the Gentile converts and that they should abstain from four things: things polluted by idols, sexual immorality, things strangled, and blood.

Act 15:20  but that we write to them to abstain from things polluted by idols, from sexual immorality, from things strangled, and from blood.

Act 15:29  that you abstain from things offered to idols, from blood, from things strangled, and from sexual immorality. If you keep yourselves from these, you will do well. Farewell.

Act 21:25  But concerning the Gentiles who believe, we have written and decided that they should observe no such thing, except that they should keep themselves from things offered to idols, from blood, from things strangled, and from sexual immorality.”

To the modern reader these four things seem strange and somewhat arbitrary.  Why were Gentile converts instructed to abstain from these four things?  Let’s take a closer look.

Acts 15:1-18 lays out how God made it clear to the apostles that salvation was open to both Jews and Gentiles.  Peter summarised the situation as follows. 

Act 15:7  And when there had been much dispute, Peter rose up and said to them: “Men and brethren, you know that a good while ago God chose among us, that by my mouth the Gentiles should hear the word of the gospel and believe.
Act 15:8  So God, who knows the heart, acknowledged them by giving them the Holy Spirit, just as He did to us,
Act 15:9  and made no distinction between us and them, purifying their hearts by faith.
Act 15:10  Now therefore, 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?
Act 15:11  But we believe that through the grace of the Lord Jesus Christ we shall be saved in the same manner as they.”

Note that Peter addressed those Jewish converts who wanted to impose on the Gentile converts various Jewish practices which Peter described as a yoke which their fathers could not bear.  This “yoke” included circumcision, but also other practices are not mentioned in this passage.

James then spoke in agreement with Peter and concluded with a list of restrictions they placed on the Gentiles.

Here are two possible explanations for the four abstentions.

Explanation 1

In verse 21 James gave the reason for the four abstentions, namely that the apostles wanted the Gentiles to have access to the Synagogues so they could hear the Old Testament being read. 

This explanation assumes the Gentile converts did not have access to a church where they could hear the Scriptures being read and that the Synagogues would bar them from entry unless they abstained from the four things.

The weakness of this explanation, in addition to the two assumptions, is that the abstentions are something the Gentile converts would be adhering to anyway (with the possible exception of eating meat offered to idols).  So what’s the point of telling them to do something they were already doing?

Explanation 2

In verse 21 James pointed out that the four abstentions are part of Scripture (and thus should not be considered burdensome).  The four practices listed in the abstentions are practices Gentile converts would have been deeply involved with in their previous pagan life, thus the instruction to abstain from them is understandable.

But why the instruction to abstain from things polluted by idols?  This seems to go against 1 Corinthians 8, where Paul explained in that it’s okay to eat food offered to idols, so long as it doesn’t cause offense to others.  Perhaps the answer is found a couple of chapters later.

1Co 10:19  What am I saying then? That an idol is anything, or what is offered to idols is anything?
1Co 10:20  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.

Idols are nothing, but sacrificing to idols is sacrificing to demons.  The Gentile converts had previously been involved in worshipping demons (perhaps unwittingly) by sacrificing and eating food offered to idols.  As a new convert coming out of paganism they needed to steer well clear of this.  The four abstentions do just that.

Conclusion

The Bible does not provide an explicit reason why the apostles decided on the four abstentions.  However, the most likely explanation is that it was to help the Gentle converts come out of the paganism they had been so steeped in.  The four abstentions are all closely tied to the pagan practises they had once observed.