In my article What is the Good News of the Bible?, I wrote the following:
There is a difference between a believer and a disciple. A believer is a person who has eternal life because he has believed in Jesus (John 6:40). All believers should become disciples, but becoming a disciple is not a requirement of receiving everlasting life. A disciple who is a believer will have an abundance of rewards in heaven (Matthew 19:27). A believer who is not a disciple will be happy in heaven, but will not have much of an inheritance there (2 Timothy 2:10-13). However, a disciple who is not a believer does not possess eternal life, and will not enter the kingdom of God until he believes in Jesus.
In the Book of Acts, all Christians are consistently referred to as disciples. At first glance, this might seem to contradict what I said above about believers and disciples being different. However, what needs to be considered is that any believer who joins a local church is automatically regarded as a disciple. Local churches must be governed by rules; otherwise, they would just fall into chaos. Therefore, anyone who wants to attend a local church must abide by these rules. This commitment to the rules is basically an initiation into discipleship. If a church member breaks the rules, he can be disciplined by the church or even excommunicated in extreme scenarios (1 Corinthians 5:13). A believer who is not interested in discipleship cannot become a member of a local church. Such a person will still have eternal life, but will certainly miss out on the privileges, rewards, benefits, and responsibilities of being a disciple. If such a person chooses a path of ungodly living, he will be disciplined by God individually (Hebrews 12:6). If such a person does not take heed to God's discipline, he could be subject to having his "conscience seared with a hot iron" (1 Timothy 4:2), "becoming entangled and overcome by the pollutions of the world" (2 Peter 2:20), or in extreme cases, subject to premature death (1 Corinthians 11:30-32). In such a case, the person would still be saved, but will surely meet with shame at the judgment seat of Christ (1 Corinthians 3:15).
What I said above about local churches, also holds true for fellowship among believers. Once fellowship is established, certain rules must be followed. These rules create a disciple-like relationship. For example, the Apostle Paul told us, "But now I have written unto you not to keep company, if any man that is called a brother be a fornicator, or covetous, or an idolater, or a railer, or a drunkard, or an extortioner; with such an one no not to eat." A believer who is not interested in fellowship still has eternal life. Nothing can impact a person's possession of eternal life. That's one of the reasons why it is called "eternal" life. It cannot be lost. However, if such a person pursues ungodly living, he will be disciplined by God as I described above.
Finally, if a believer wants to have personal fellowship with God, he must do the things that please God. This is the major theme of the letter of 1 John. In the letter's purpose statement, we read the following: "3 That which we have seen and heard declare we unto you, that ye also may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ. 4 And these things write we unto you, that your joy may be full" (1 John 1:3-4). A believer who is out of fellowship with God still has eternal life, but if he chooses to pursue ungodly living, he will be in danger of suffering the discipline I described above.
To conclude, I would just like to reiterate that all believers should become disciples. However, becoming a disciple is not a requirement of receiving eternal life. At the same time, however, if a believer refuses to become a disciple and obstinately pursues ungodly living, such a person will be destined for discipline from God. In contrast, all disciples should examine themselves to make sure they are in fact believers. A disciple of Jesus Christ who has not yet believed in Jesus Christ for eternal life does not yet possess eternal life. In such a case, the person may still be ignorant of the Gospel. Or, as I suspect in many cases, the person may have chosen to try to earn eternal life through his life of discipleship and religious works. In any case, an non-believing disciple is infinitely worse off than a non-discipled believer. The best course of action would be to become a discipled believer. With that said, let us all believe the Gospel and pursue a life of discipleship.