Jesus promised his discples that the Holy Spirit would lead them to author what would be known as the New Testament
The Holy Spirit will teach you and remind you of everything - John 14:25-26
He will guide you into all truth - John 16:12-13
Message received by them:
Thrity five miracles in the book of Acts alone - The miracles confirmed the message. God's way of saying the message was really coming from him.
Made up? Not likely since they were often beaten, tortured, and killed for affirming them
The miracles never benefited directly or for entertainment. When the New Testament was completely written, the miracles stopped too (about 62AD)