What does Luke 3:21 mean?

Curiously, Luke does not give details about Jesus' baptism; he simply notes that it happened. The other three gospel writers give more time and detail to the event. Since Mark (Mark 1:9–11) and Matthew (Matthew 3:13–17) produced their accounts before Luke's, it's possible Luke felt their description of Jesus' baptism was enough. The simple phrase "when Jesus also had been baptized" implies the original reader—established early on as a believer (Luke 1:3–4)—had some knowledge of the event already. The only details Luke offers (Luke 3:22) are those that continue to establish the point made by John the Baptist: that Jesus is the Promised One sent by God (Luke 3:16–17).

Luke uses the expression "all the people" in reference to those baptized. This is the same style used by Matthew (Matthew 3:5–6) and Mark (Mark 1:5). This is not a literal claim that every single living person was baptized before Jesus. Rather, it's a reference to John's fame in that region (Mark 11:29–32). In this case, it suggests Jesus' baptism occurred at the height of John the Baptist's popularity.