What does Matthew 6:18 mean?
This statement is very similar to comments Jesus made about charity (Matthew 6:4) and prayer (Matthew 6:6). Jesus promised that God the Father sees what His people do in secret. This was not a threat or a warning. His point was that God notices when His faithful people meet the needs of others, or pray or, as stated now, participate in fasting. God is watching and sees when His people honor Him in their hearts even if nobody else ever knows it. Since He is the only One we should seek to please, He is the only One who needs to know.Jesus goes even further: Not only does God see; He rewards such behavior. Jesus emphasizes over and over in His teaching that God truly cares more about the hearts and motivations of His people than their outward actions. This is a major theme of the Sermon on the Mount, and especially the teachings given in chapter 5.
It's hard to estimate how this teaching must have affected the original audience. These people had lived their entire lives under a performance-driven religious system where all that mattered was outward actions. Image, reputation, honor and shame were all that mattered. That's not how the law given by God was supposed to work (Matthew 5:17), but that was how it had come to be practiced. Jesus is correcting the teaching and practice of Israel's religious leaders (Matthew 5:20).