And we are witnesses - We who are apostles. See the notes on Luke 24:48.
In the land of the Jews - In the country of Judea.
Whom they slew ... - Our translation would seem to imply that there were two separate acts - first executing him, and then suspending him. But this is neither according to truth nor to the Greek text. The original is simply, “whom they put to death, suspending him on a tree.”
On a tree - On a cross. See the notes on Acts 5:30.