Mat 5:18 “For truly I say unto you…” Here, for the first time, does that august expression occur in our Lord’s recorded teaching, with which we have grown so familiar as hardly to reflect on its full import. It is the expression manifestly of supreme legislative authority: and as the subject in connection with which it is uttered is the Moral Law, no higher claim to authority strictly divine could be advanced. For when we observe how jealously Jehovah asserts it as his exclusive prerogative to give law men (Leviticus 18:1-5, 19:37, 26:1-4, 13-16, etc.) such language as this of our Lord will appear totally unsuitable and indeed abhorrent, from any creature-lips.”
Jamieson, Fausset, & Brown