Acts 16:4

And as they went on their way through the cities, they delivered them the decrees to keep, which had been ordained of the apostles and elders that were at Jerusalem.

The essential message of those decrees was that Gentiles were not to be burdened by circumcision and law-keeping; and by providing copies of them for the young churches, Paul protected them against the devices of the Judaizers. This was the position Paul had required the apostles and elders in Jerusalem to accept. Even the four prohibitions regarding idols, fornication, blood and things strangled were grounded not in the law of Moses primarily, but in God's teachings which antedated the Mosaic covenant (Genesis 9:3-5).