Why were the religious leaders familiar with the Psalms?
The religious leaders were familiar with the Psalms because they knew their Old Testament, especially the Psalms, and sang them in their worship. They knew the Psalms as intimately as they knew their own names, which is why Stephen's words about Jesus standing at God's right hand provoked their anger.
Les explains that Stephen was speaking to the religious leaders, who knew their Old Testament, especially the Psalms. They sang the Psalms in their worship and knew them like they knew their own names. Les suggests that something attached to Stephen seeing Jesus standing at God's right hand aroused their anger. He points to Psalms 67 and 68 as possibly explaining why, and says this is the explanation he is comfortable with. Their deep familiarity with the Psalms meant Stephen's words struck a chord with them.