During the first century, the Bible was not yet compiled. So what was the basis of Christianity during that time?
"For this reason we also constantly thank God that when you received the word of God which you heard from us, you accepted it not as the word of men, but for what it really is, the word of God, which also performs its work in you who believe" (I Thessalonians 2:13).
"So then, brethren, stand firm and hold to the traditions which you were taught, whether by word of mouth or by letter from us" (II Thessalonians 2:15).
The early church had the apostles and prophets to teach Christians God's commands. "And He gave some as apostles, and some as prophets, and some as evangelists, and some as pastors and teachers, for the equipping of the saints for the work of service, to the building up of the body of Christ" (Ephesians 4:11-12). While these teachings were only partial (I Corinthians 13:8-10), they were enough to get the early church started until the parts of the New Testament could be written. The New Testament was written relatively rapidly in about 50 years. But in the intervening years, what was written was copied and past around between the churches (Ephesians 3:4-7; Colossians 4:16; I Thessalonians 5:27).