Then it seemed good to the apostles and the elders, with the whole church, to choose men from among them and send them to Antioch with Paul and Barnabas. They sent Judas called Barsabbas, and Silas, leading men among the brothers, with the following letter: “The brothers, both the apostles and the elders, to the brothers who are of the Gentiles in Antioch and Syria and Cilicia, greetings. Since we have heard that some persons have gone out from us and troubled you with words, unsettling your minds, although we gave them no instructions, it has seemed good to us, having come to one accord, to choose men and send them to you with our beloved Barnabas and Paul, men who have risked their lives for the name of our Lord Jesus Christ. We have therefore sent Judas and Silas, who themselves will tell you the same things by word of mouth. For it has seemed good to the Holy Spirit and to us to lay on you no greater burden than these requirements: that you abstain from what has been sacrificed to idols, and from blood, and from what has been strangled, and from sexual immorality. If you keep yourselves from these, you will do well. Farewell.”
So when they were sent off, they went down to Antioch, and having gathered the congregation together, they delivered the letter. And when they had read it, they rejoiced because of its encouragement. And Judas and Silas, who were themselves prophets, encouraged and strengthened the brothers with many words. And after they had spent some time, they were sent off in peace by the brothers to those who had sent them. But Paul and Barnabas remained in Antioch, teaching and preaching the word of the Lord, with many others also. (Acts 15:22-35)
It was agreed not to force new Gentile believers in Jesus to become Jews. But the rules chosen were such as to avoid habits that would be very offensive to the consciences of Jews and Jewish Christians; for many in their towns did strictly observe the Mosaic laws about idols and blood.
All Christians would agree, of course, on the basic laws like ‘no killing’ , but, as to sexual matters, the need for purity was given emphasis. This was because sexual immorality was so widespread in the Gentile culture, and even formed part of temple worship.
In most parts of our modern world too it would seem that a special rule of sexual purity is needed.