C. F. Keil and Franz Delitzsch on Isaiah 57:14

Isa 57:14

The promise is now followed by a appeal to make ready the way which the redeemed people have to take. “And He saith, Heap up, heap up, prepare a way, take away every obstruction from the way of my people.” This is the very same appeal which occurs once in all three books of these prophecies (Isa 40:3-4; Isa 57:14; Isa 62:10). The subject of the verb ('âmar) is not Jehovah; but the prophet intentionally leaves it obscure, as in Isa 40:3, Isa 40:6 (cf., Isa 26:2). It is a heavenly cry; and the crier is not to be more precisely named.