In the first few sentences of Acts, Luke said that in his former book (which we now call the Gospel of Luke), he wrote about all that Jesus began to do and teach… (Acts 1:1 NIV). One thing this means is that the book we now call Acts (Luke’s second book) is a record of what Jesus continued to do and teach even though he had ascended into heaven. As John Murray wrote, Jesus “is ever active in the exercise of his prophetic, priestly, and kingly offices.”
The fact that Jesus continued to teach after his ascension is of paramount importance for the authority of Christ in the teaching of the apostles and in the books of the New Testament. Prior to his ascension Christ’s teaching was directly by word of mouth. But afterwards he taught by a different mode. He taught by the ministry of appointed witnesses and inspired writers. The New Testament, all of which was written after Jesus’ ascension, is not one whit less the teaching of our Lord than that delivered verbally during the days of his flesh. How utterly false it is to set up a contrast between the authority of Jesus’ spoken words and the authority of the New Testament as Scripture. The latter is the teaching of Christ given in his own appointed way after his ascension.
We are reminded of Jesus’ word to the disciples: ‘I have yet many things to say to you, but you cannot bear them now. Howbeit when he, the Spirit of truth, comes he will guide you into all truth’ (John 16:12, 13). It is from his own lips the certification of Luke’s statement in our text (Acts 1:1-2). The guiding of the Holy Spirit into all truth does not suspend Jesus’ own speaking. ‘I have yet many things to say to you.’ But he says these things through the Holy Spirit and thus there is the seal of both divine persons, the Son and the Spirit.
So we don’t need a red-letter Bible, nor do we need to put Jesus’ spoken words on a higher level than the Spirit-inspired words of Paul (or the other human authors of the New Testament)! Murray ends the paragraph like this:
Let us prize with the ardor of our soul what Jesus continues to do, and teach. He is the living, acting, and teaching Lord.
John Murray, Collected Writings, volume 1, pages 41-41.