The Pharisees continued to oppose Jesus (2:16–18, 24; 3:6; 7:1–5; 10:2; 12:13, 15). Their hostility did not stop.
Sometimes, asking for a sign is acceptable (Judges 6:36–40; 2 Kings 20:8–11; Isaiah 7:10–12; see also John 2:18–19). But in this case, their request was not sincere. It was a stubborn test of God (they were trying to challenge or trap him).
They were not asking for the kinds of signs Jesus had already done, such as healings, driving out demons (evil spirits), raising the dead, or controlling nature. Many of these had already happened, and people widely knew about them (Mark 1:32–34, 45; 3:7–12; 6:53–56). Even the Pharisees knew about them (3:22).
Instead, they wanted a sign from heaven (that is, a direct sign from God) to prove once for all that Jesus is the Messiah (God’s chosen king). But no sign would have convinced them.
For those who were open to the truth, the miracles Jesus had already done were clear signs that he is the Messiah, the Son of God (Luke 7:22). But for those who were not open, no sign would lead them to believe (Mark 4:11–12; compare Luke 6:19–31).