Acts 15 is such an amazing chapter that it deserves several posts. And even then we will only scratch the surface with observations rather than real depth. Remember the first rule of reading blogs: books are better. And immersing yourself wholeheartedly in the Bible is infinitely better than both (though not mutually exclusive). If you have a choice right now between either reading this post or reading Acts 15 (or any other part of the Bible), go do that instead.
Remember that the message that has been preached to Jew and Gentile on the first missionary journey is the good news of the grace of God (eg 14:3), and especially about the resurrection (13:32-3). The blockbusting implication for its first hearers was that through Jesus forgiveness of sins is available and everyone who believes is justified from everything that they couldn't be justified from by the Law of Moses (13:39).
This last point was bound to cause a bustup sooner or later, because it reveals that to become a believer in the Lord and be saved you didn't have to become a Jew first. And more provocatively still, the thing that you thought as a Jew brought you into covenant relationsip with God (and thereby saved you), didn't. They taught that obedience to the Law of Moses, to God's law, was irrelevant to the question of how to get saved. In Acts 15 the issue comes to a head.
Some people came (unauthorised as it happens), from Jerusalem to Antioch and taught that to be a believer - to be justified - you also had to become a Jew, with all that meant for Law-obedience and circumcision. Paul and Barnabas sharply disagreed. They went up to Jerusalem to see the Jerusalem apostles and get the matter resolved once and for all.
v5 is the critical statement of the issue: some believers who belonged to the party of the Pharisees stood up and said "the Gentiles must be circumcised and required to obey the Law of Moses."
Direct contrast: do we need to obey the Law to be justified, or is it entirely by grace alone?
A few more observations:
1. There were - and are - believers who think that either justification or sanctification or both depend on obedience to the Law of Moses. This passage talks only of justification. The question of whether we are sanctified by grace alone is not dealt with here (we are!). This was not a secondary matter on which difference of opinion was legitimate. Though dealt with generously the matter had to be settled, or else there would be wrong teaching about the way of salvation, wrong practise in the churches, and believers who were not liberated to enjoy being children of God.
2. There are many issues in church life where it is best to agree to disagree, with a generous spirit. The number of hills on which we should really choose to die is comparatively few. Too many people want to make every minor issue into a primary and divide churches over trivia. But primaries do exist. This is definitely one of those hills to die on. Beware being so generous in agreeing to disagree in churches that we also negotiate on the primaries. This is a classic case of the need to defend gospel primaries till the bitter end, while being clear what those primaries are and what are secondaries.
3. I have had a good number of Catholic friends in my time and, under various circumstances, enjoyed fellowship with them in some projects. I have evangelical friends today who have no idea of official Catholic doctrine and therefore cannot understand why I make salvation by grace out to be a primary matter that stands between me and Catholic friends, meaning I cannot work with them in evangelism. The reason is Acts 15. What is at stake is nothing less than two contradictory understandings of how we get saved - is it by grace alone or some other way? There is no detente to be had on this issue (there is on other issues). There is no quarter to be given. It is by grace alone, sovereignly from God, with me contributing nothing, only gratefully believing and throwing myself on his grace. Period.
4. Note that there were people in the church from the party of the Pharisees. And they were believers. They were wrong and needed to be corrected on a vital issue of first importance, but they believed. Let's resist the temptation to assume too quickly that people in the church who believe wrong things don't belong to Jesus. They didn't know they were in by grace alone, but they were! You can even get this issue wrong and have God be gracious to you - that's how big grace is.