Why didn't Jesus preach the gospel of grace during His earthly ministry?
During His earthly ministry, Jesus did not preach the gospel of grace because His death, burial, and resurrection had not yet taken place. The gospel of grace required that finished work, and it could not be proclaimed until God revealed it. Instead, His instructions conformed to the Mosaic system.
Les explains that everything Jesus said and instructed during His earthly ministry was in compliance with the Mosaic system, not the gospel of grace. It could not be otherwise, because Christ had not yet died, and His death, burial, and resurrection had to be completed before that gospel could be preached. Les points to Luke 18:31, where Jesus told the Twelve they were going up to Jerusalem and all things written by the prophets concerning the Son of Man would be accomplished; it had not happened yet, but it would. You cannot preach something until God reveals it, and He had not yet revealed it. This is why Les emphasizes that continuing to preach only in the four gospels while ignoring the Apostle Paul misses that the death, burial, and resurrection had not yet taken place.