The Bible was HOLY SPIRIT INSPIRED. NOT word of mouth!
In the beginning was the Word, and the Word was with God, and the Word was God. John 1:1
Of course it was word of mouth for the longest time. The gospels weren’t written until 60 to 100 years after the crucifixion. There are written accounts from the Roman and Hebrew scribes of the time. The Romans kept meticulous records of their activities.