Why was Saint Stephen a martyr?

A:

Saint Stephen is only known from Acts of the Apostles, which tells us that Stephen was interrogated because of his success in preaching to the Jews, although why they chose to try Stephen rather than Peter or any of the other apostles is not made clear. However, this was not real reson for his martyrdom.

Rather than defend himself against the charges made against him, Acts says that Stephen engaged in a monologue, surely designed simply to insult the Jews. When he saw that this angered the priests who were trying him, Stephen looked upwards and saw heaven open and Jesus standing on the right hand of God. Apparently the others could not see what Stephen saw, and cast him out of the city, to be stoned to death.

We now know that what Stephen saw was scientifically impossible, yet Acts gives this as the immediate reason for his martyrdom. Moreover, Raymond E. Brown (An Introduction to the New Testament) says that we can never verify the martyrdom or even existence of Stephen.