The Pharisees were getting frustrated. Jesus had told his disciples to have nothing to do with the blind guides. The Pharisees can’t best Jesus in their thinking or knowledge of God’s word, so they asked for a sign.
Realize that some of the Pharisees had seen the mighty acts of God delivered through Jesus and sometimes through Jesus and his disciples. So why do they ask for a sign?
The Pharisees want Jesus to perform for them. They want a miracle on demand. They want Jesus to recognize their status and comply with their request for a sign.
Then Jesus dives into a nautical piece of wisdom. Red sky in the morning, sailor take warning. Red sky at night, sailor’s delight. OK, the nautical saying came later surely in concert with the words of Jesus.
Jesus told these religious hypocrites that they understood the signs of the day, but not the signs of God. The men who were to have known God more than everyone else, knew him the least.
Jesus noted that it was a sinful generation that sought a sign. A sinful generation was blind to the mighty acts of God at work in the world and the Son of God standing before them.
Jesus has already noted that the Pharisees were the blind leading the blind.