There is no such thing as a believer who doesn't bring forth good fruit. For when ye were the servants of sin, ye were free from righteousness. The implied answer is none, because they weren't saved. Any fruit they had was the fruit of sin. But now being made free from sin, and become servants to God, ye have your fruit unto holiness, and the end everlasting life.
Every believer has everlasting life, which is the culmination of a righteous life.
Therefore people who don't bear fruit cannot be believers. That sounds like the people who don't bear fruit are Christians because of their association with Christ. Some relationships to Christ are in appearance only. An individual can apparently seem connected to Jesus Christ but, in fact, not be connected at all. The apostle Paul warned against being superficially attached to Christ.