Peter, Paul, and other first-century disciples were Christians, but what denomination were they members of? None at all! Denominational division, as we know it, did not exist in the first century. All Christians were in one body — the church which belongs to Jesus (Matt. 16:18; Eph. 4:3-6). First-century Christians were members of Jesus' church, but not members of any denomination. Why can't you and I do the same today?
In fact, is Jesus even pleased with the existence of so many different denominations? In John 17:20-21, Jesus prayed concerning those who believe in Him, "that they all may be one; as thou, Father, art in me, and I in thee." In denominationalism today, are all people one as the Father and Son are one? Surely not.
So, how can we be just Christians today without contributing to denominational division? Well, how did people do it in the first century? They obeyed the teachings of Jesus given in the gospel. Obedience to the gospel cleansed them from sin, and then the Lord added them to His church (Acts 2:36-41,47). If you and I follow the same gospel, won't the same thing happen to us (see Acts 2:39)?
But suppose instead we follow the creed book or church law of some denomination. What then would we become? We would become members of that denomination, right? Or if we followed the laws of some other denomination, we would become members of that denomination. However, following just the New Testament would make us simply children of God, just like it did in the first century (1 Peter 1:22,23; Gal. 3:26,27). Denominational doctrines make denominational members. The New Testament makes just Christians.