"And certain men came down from Judea and taught the brethren, 'Unless you are circumcised according to the custom of Moses, you cannot be saved.' Therefore, when Paul and Barnabas had no small dissension and dispute with them, they determined that Paul and Barnabas and certain others of them should go up to Jerusalem, to the apostles and elders, about this question. So, being sent on their way by the church, they passed through Phoenicia and Samaria, describing the conversion of the Gentiles; and they caused great joy to all the brethren. And when they had come to Jerusalem, they were received by the church and the apostles and the elders; and they reported all things that God had done with them. But some of the sect of the Pharisees who believed rose up, saying, 'It is necessary to circumcise them, and to command them to keep the law of Moses'" (Acts 15:1-5).
After Paul and Barnabas had returned to Antioch in Syria from their missionary journey, a significant controversy erupted from within the church. Some of the Jewish Christians had been teaching that Gentile converts to Christ must, in addition to being baptized, also be circumcised and keep the other tenets of the Law of Moses. They affirmed that a Gentile could not be saved without circumcision, even if he had obeyed the gospel of Christ! Once these teachers and their message spread to Antioch, it met immediate and strong resistance from Paul and Barnabas. The dispute over the matter became so great that it was decided that Paul, Barnabas, and some others would travel to Jerusalem to meet with the apostles and elders there regarding this matter. Let it be observed that the behavior of Paul and Barnabas here teaches us that it is right to contend for the truth of the gospel regardless of the debate that may ensue. It is never right to compromise doctrine in an effort to achieve "peace."
On their way to Jerusalem, Paul and Barnabas spread the good news of their successful outreach to Gentiles. This caused the brethren to rejoice and praise God! However, after arriving in Jerusalem and being well-received, some of the Christians who were also Pharisees spoke against their evangelistic efforts. They maintained that it was necessary for these Gentiles who had believed and obeyed Jesus' gospel to also be circumcised and keep the Old Law. In other words, they believed the efforts of Paul and Barnabas were incomplete and in violation of the Old Testament. The question regarding whether or not Gentiles could become Christians had already been settled (cf. Acts 10), but now the question had essentially become: "What should the relationship of a Christian be to the Old Testament?"
Although the specifics of this controversy may seem unimportant to us, it was of great significance to the early church and Luke devotes much of Acts 15 to the problem and its divine solution. Even to this day, the larger question is still very important and often misunderstood. Further information pertaining to this dispute may be gleaned from Galatians 2:1-10, which I believe to be a parallel passage to what is recorded here in Acts 15.