The Gospel of Mark was written around 70 AD, the first gospel to be recorded. The other three came 15-30 years later. How were they followers of Jesus before those were written down? They celebrated the Eucharist, honored the saints (offering Mass on their tombs in the catacombs, etc.), celebrated the sacraments, basically lived as Catholics. The writings of the very early Church Fathers show these practices plainly. Acts and the Letters of St. Paul were being written meanwhile and also refer to early Christian life that is very familiar to Catholics and not at all opposed to our belief and practice.

Then the Catholic Church's actual canon of the Bible was established through a series of councils and synods over several centuries: Council of Rome: In 382 AD, Pope Damasus I presided over the Council of Rome, which promulgated the canon of the Bible in a document called the Decree of the Council of Rome on the Canon of Scripture. Synod of Hippo: In 393 AD, the Synod of Hippo affirmed the canon of Scripture. Councils of Carthage: The Councils of Carthage in 397 and 419 AD also affirmed the canon of Scripture. Council of Florence: The Council of Florence from 1431–1449 reaffirmed the canon of Scripture. Council of Trent: The Council of Trent from 1545–1563 defined the canon of Scripture as an article of faith. The Catholic Bible has 73 books in total, including 46 books in the Old Testament and 27 books in the New Testament. The seven books in the Catholic Old Testament that were removed by Martin Luther starting in the 16th century are Tobit, Judith, 1 and 2 Maccabees, Wisdom, Sirach, and Baruch. Not sure who gave him that authority...

The Gospel of Mark was written around 70 AD. How were they followers of Jesus before that was written down? They celebrated the Eucharist, honored the saints (offering Mass on their tombs in the catacombs, etc.), celebrated the sacraments, basically lived as Catholics. The writings of the very early Church Fathers show these practices plainly.