Why does the teacher say the Hebrews' rejection of Moses is laying the groundwork?
The Hebrews' rejection of Moses, asking who made him ruler, foreshadows Israel's rejection of Jesus as King. When Jesus presented Himself as Israel's king, they similarly asked who made Him ruler and declared they had no king but Caesar. This pattern lays groundwork for their rejection of Christ.
Les teaches that when the Hebrew asked Moses, "Who made thee a prince and a judge over us?" (s403-s404), it should ring a bell. He points out that this same statement appears in the New Testament when Jesus presented Himself as Israel's king. On the morning of His trial, Israel said, "Who made you a ruler over us?" (s409-s410). When the Roman authorities asked Israel if He was their king, they answered, "We have no king but Caesar" (s413-s414). Les explains that all of this is laying the groundwork (s415-s416). Thus, the Hebrews rejected Moses and had nothing to do with him because they said, "Who made you to be a ruler over us?" (s417-s419). This rejection prefigures Israel's later rejection of Christ as their king.