You tell us what happened to word of mouth.....
What is it and how do you know?
How do you know that it is actual teaching from the apostles and not something added later that someone is CLAIMING is from the apostles?
Because Jesus told me so.
We have Christ’s promise that His Church will always teach the Truth:
“And I say also unto thee, That thou art Peter, and upon this rock I will build my church; and the gates of hell shall not prevail against it.” Mat 6:18
“I have yet many things to say unto you, but ye cannot bear them now. Howbeit when he, the Spirit of truth, is come, he will guide you into all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come.” John 16:12-13
“But if I tarry long, that thou mayest know how thou oughtest to behave thyself in the house of God, which is the church of the living God, the pillar and foundation of the truth.” 1Tim 3:15
I believe Scripture. Do you?