“But there rose up certain of the sect of the Pharisees which believed, saying, That it was needful to circumcise them, and to command them to keep the law of Moses.” (Acts 15:5)
It is interesting here that these believers had very bad doctrine concerning soteriology. This would be a passage to remember whenever someone tries to say that some man greatly used by God in history (Martin Luther, etc.) is not saved.
We don’t know whether someone is saved or not, but these believing Pharisees thought that Gentiles had to keep the law to be saved, yet they are still called believers.
Interesting! Maybe God’s grace is sufficient, and strict wooden theology is not.
You Think Too Much! 
Yes, they believed; therefore, they knew salvation was by faith alone. However, they were requiring some now extra-Scriptural behavior in light of a new dispensation, very shortly after the former one. We probably shouldn’t conclude that they had poor soteriology. I don’t think you can make that conclusion from that text. If they were adding to grace, Christ would be of no effect unto them (Galatians 5:1-4). These were some of the Pharisees who had been saved, ones who motivated the Jerusalem council designed to sort through these issues in Christian living. Nothing was wrong with circumcision or keeping the ceremonial laws of Moses; Paul chose to keep many of these himself for the sake of evangelism. A good application from this text is that people who believe in Christ will subject to changes in fitting with Scripture and church authority. I believe that Martin Luther went to his death adding infant sprinkling to grace. That doesn’t mean that we shouldn’t recognize his historical contributions.