Charles Ellicott Commentary Isaiah 58:1

"Cry aloud, spare not, lift up thy voice like a trumpet, and declare unto my people their transgression, and to the house of Jacob their sins." — Isaiah 58:1 (ASV)

Cry aloud ... —Literally, with the throat, that is, with no faint whisper as from stammering lips, but with full strength of voice. The work of the preacher of repentance is not to be done slightly or by speaking smooth things . The “trumpet” of the next clause emphasises the thought yet further.