In Acts 15 we read about the conference that addressed the conversion of Gentiles. Up until this time, the church was exclusively Jewish, and it is doubtful that any Christians had really absorbed into their psyche that any of the "nations", the Gentiles, could be part of God's people unless they converted to Judaism.
The Jews did not, however, consign all Gentiles to being lost. Whereas the Jews believed they were under a special covenant with God, which included obedience to the 613 laws of the Mosaic Covenant, they believed that "good" Gentiles who obeyed the seven laws of the Noahic Covenant would find a place in paradise. These seven laws, derived from the covenant God made with Noah after the Flood, applied to all Nations, whereas the 613 laws of the Mosaic Covenant only applied to the Jews.
The result of this conference in Acts 15 was that the Gentile believers did not need to submit to the Law of Moses the way the Jewish believers did, because the Gentiles were not converting to the Mosaic Covenant, which was ritualistic and for the Jews only, but rather to the Abrahamic Covenant, which was by faith and was for all nations, and which was fulfilled in Jesus.
Nevertheless, to keep social peace with the many Moses-observing Jews living in most cities, the Gentiles needed to adhere to the seven laws of the Noahic Covenant which are:
1) to establish courts of justice;
2) to not commit blasphemy;
3) to not commit idolatry;
4) to not commit incest and adultery;
5) to not commit bloodshed;
6) to not commit robbery; and
7) to not eat flesh cut from a living animal.
The instructions in Acts 15 were:
1) to abstain from food sacrificed to idols;
2) to abstain from blood;
3) to abstain from the meat of strangled animals; and
4) to abstain from sexual immorality.
I suspect that the Jerusalem Conference members felt that these four rules were sufficient to appease the Jews living amongst them as being "close enough" to the seven Noahic Laws. The reason these rules were given to the Gentiles is because "... Moses has been preached in every city from the earliest times and is read in the synagogues on every Sabbath" (v21), and that the new Gentile Christians would "do well to avoid these things" (v 29).
You can read more about the Jewish conception of the Noahic Law for Gentiles here: http://www.jewfaq.org/gentiles.htm