The Mekhilta draws a parallel that cuts both ways. In the previous passage, the rabbis established that believing in Moses equals believing in God. Now they demonstrate the reverse: speaking against Moses equals speaking against God.
The proof text comes from the wilderness: "And the people spoke against God and against Moses" (Numbers 21:5). The Mekhilta applies the same logic as before but in the negative direction. If the Israelites spoke against God, it would be obvious that they also spoke against Moses — the servant is a lesser target than the master. So why does the verse mention Moses by name alongside God?
The answer mirrors the earlier teaching with devastating precision. We are hereby apprised that one who speaks against the "faithful shepherd" speaks against Him who spoke and brought the world into being. The principle works identically in both directions. Faith in the prophet is faith in God. Rebellion against the prophet is rebellion against God. There is no neutral ground.
The Mekhilta establishes that in Jewish theology, prophetic authority is not separate from divine authority — it is an extension of it. Moses does not speak on his own behalf. His words carry the full weight of the Creator. To grumble against Moses in the wilderness was not merely disrespecting a human leader. It was an act of defiance against the divine source of his mission. The mouth that complains about the prophet is, in truth, complaining about God.