Orally passing on the truths of Christianity would not have been effective without the inspiration,teaching,prophecy and power of the Holy Spirit as this verse indicates:
John 16:13 (Contemporary English Version)
The Spirit shows what is true and will come and guide you into the full truth. The Spirit doesn't speak on his own. He will tell you only what he has heard from me, and he will let you know what is going to happen.
These words of Jesus were fulfilled after his murder when as promised He sent His Holy Spirit in such a miraculous way that the event is still celebrated in most churches on what is now called "Pentecost Sunday".
Acts 2:1-4 (Contemporary English Version)
The Coming of the Holy Spirit
1On the day of Pentecost all the Lord's followers were together in one place. 2 Suddenly there was a noise from heaven like the sound of a mighty wind! It filled the house where they were meeting. 3Then they saw what looked like fiery tongues moving in all directions, and a tongue came and settled on each person there. 4The Holy Spirit took control of everyone, and they began speaking whatever languages the Spirit let them speak.