Variantly: "Would you thus requite the L-rd?" An analogy: one is standing and taunting his father. A bystander: Who are you taunting? Your father? How much toil did he expend on you! How much strength! If you did not honor him in the past, (at least) you should honor him now, so that he not write over all of his holdings to others! Similarly, Moses said to Israel: If you do not remember the miracles and mighty acts that the L-rd performed for you in Egypt, (at least) be mindful all the good that He is destined to bestow upon you in the world to come (if you heed His word).