Why is the transformation of the disciples evidence for Jesus' resurrection?

Answer 1

Our Blessed Lord told the disciples that after His resurrection and ascension, He would send them the Holy Spirit, who would strengthen them and guide them. The transformation of the disciples thus was evidence for the arrival of the Holy Spirit, and by extension, the evidence that Our Blessed Lord had ascended to heaven and sent Him as He said He would.

Answer 2

The transformation of the disciples would be evidence for Jesus' resurrection if that transformation really occurred. For this we have only two sources - Acts of the Apostles and tradition.

Uta Ranke-Heinemann (Putting Away Childish Things) says Acts of the Apostles is a work of Propaganda. This is hardly a ringing endorsement for the claims made in the book, including that the Holy Spirit came down and transformed the apostles (Acts chapter 2). She also cites the German theologian, Hans Joachim Schoeps, who writes that Acts has been "believed much too readily.” This is changing and, in recent decades, the consensus of critical scholars is that Acts can not be relied on as a history of the early Church.
Tradition says that all the apostles were ready to give up their lives in order to preach the gospel and that they did so, all except John son of Zebedee. In fact tradition implies that some of the apostles lost their lives more than once, in different places and by different means. Not one of these martyrologies can can be confirmed, so we have to dismiss them as legendary.

The purported transformation of the disciples is insufficient evidence to provide any support for the resurrection and ascension of Jesus.