“And of which of you who is a father will his son ask a fish, and he for a fish give him a serpent? Or if he shall ask an egg, will he give him a scorpion?”
And the certainty that we will receive lies in the nature of fatherhood. When a son asks for a fish will his father give him a serpent, if he asks for an egg will he give him a scorpion? Both are inconceivable. The idea is of a mythical son (who does not exist) who thinks that he is receiving an edible eel from his father, and discovers it is a snake. He accepts what seems to be an egg, but it unwinds itself as a scorpion. He has been utterly deceived, and as a result subjected to great harm. Does anyone know of a father who would do such a thing to his son? Of course not!
Here then we have the certainty that they will receive the good things they ask for. They will receive a fish or an egg (the daily food they have asked for, and common foods in Palestine), which in Luke 11:13 turns out to be the Holy Spirit. What they will not be given is spirits of evil, for both serpent and scorpion represent the power of the Enemy (Luke 10:19), and the Father will protect them from that (Luke 11:14-26). So those who can call God Father, because He has been made known to them by the Son (Luke 10:22), can seek His Holy Spirit with the absolute guarantee of success. And they can go on receiving more and more of Him.
The continual cross references in this whole section are interesting and significant, as with the serpent and scorpion here as compared with Luke 10:19. The section has very much to be seen as a whole, interconnecting and arrayed around the Lord's Prayer.