What does Matthew 9:15 mean?
Jesus is answering a question from the disciples of John the Baptist and the Pharisees. They want to know why His disciples do not fast, whereas these other groups do (Matthew 9:14). The law of Moses required fasting only once a year (Leviticus 23:26–32), but scholars believe the Pharisees fasted twice a week. It was one of many outward signs of their assumed righteousness. John the Baptist's disciples may have been following his example of a very restrictive lifestyle, though John himself was currently in prison (Matthew 4:12).Jesus' answers to direct questions often require some thought in order to understand. Rather than answering shallowly, or accepting assumptions, Christ often replied to the deeper issues at hand. In response to this question (Matthew 9:14), Jesus asks a question of His own: would you expect wedding guests to mourn while they're with the groom? He explains that His disciples will fast, eventually, when the bridegroom is taken away.
Looking back after Jesus had been fully revealed as the Messiah and the Son of God, after His death and resurrection and return to heaven, this illustration comes into clear focus.
Matthew has already quoted Jesus referring to the great banquet in the kingdom of heaven (Matthew 8:12). This banquet is sometimes called the "marriage supper of the Lamb" (Revelation 19:9). This feast represents the wedding of the bridegroom, Jesus, to His beloved church, all those who have come to Him by faith. It is not yet time for that wedding to take place, but the bridegroom, Jesus, was on earth making preparations. Why would his friends fast while the bridegroom is with them?
In the Old Testament, God Himself is described as the bridegroom (Isaiah 54:5–6; Hosea 2:16–20). The Pharisees may have caught this connection to Jesus' claim and resented the implications. Perhaps John the Baptist's disciples were more open to it since John Himself had used this language to describe his relationship with the Messiah: "The one who has the bride is the bridegroom. The friend of the bridegroom, who stands and hears him, rejoices greatly at the bridegroom's voice. Therefore this joy of mine is now complete" (John 3:29).
John the Baptist clearly understood Jesus to be the bridegroom and himself the bridegroom's friend, or what western culture would call the "best man. "