Early Christians had only bits and pieces of the Old Testament. There was no such thing as the bible, only collections of hand-copied scripture in varying assemblages. Further, the vast majority of the world’s people were illiterate. We tend to use today as the yardstick for thousands of years ago.
We refer in the present discussion to the New Testament, the Gospel of Christ. It was not formally canonized until the late 300s. John did not write his Gospel until nearly the year 100. Hebrews, not being signed, was argued over for centuries. We are certainly delighted that we have scripture, but we would still have Christ’s church even if the New Testament had never been written. “Bible” churches depend 100% on the bible, yet disagree substantially over the meaning of scripture. How does this serve unity?
Praise God that we have both tradition and scripture. What would the faith look like without both, to provide the needed stability.