Isaiah 7:4
And say unto him, Take heed, and be quiet; fear not, neither be fainthearted for the two tails of these smoking firebrands, for the fierce anger of Rezin with Syria, and of the son of Remaliah. And say unto him, Take heed, and be quiet; fear not, neither be fainthearted for the two tails of these smoking firebrands, for the fierce anger of Rezin with Syria, and of the son of Remaliah.
Judah's repeated sin had left its leaders weak and afraid of God's judgments, which explains their fear. To the fearful King Ahaz came God's message commanding him to take heed, to stay alert yet quiet and calm, and neither to fear nor to lose heart. The two tails of smoking firebrands were Rezin with Syria and Pekah, the son of Remaliah, with Israel, joined in alliance and fierce anger against Judah. Calling them smoking firebrands meant their threats looked dangerous but were nearly spent, and their anger could not overturn God's purpose. God promised that the enemies' plan against Judah would not succeed and their kingdoms would soon come to an end, so Ahaz must trust God to stand firm, for without belief he would not be established.
The verse belongs with Isaiah 6, Jeremiah 7, Jeremiah 8, and Isaiah 55:1-3 as one united word of God to backsliders. That word calls them to stop trusting deceptive words and to amend their ways and doings. It urges them to obey His voice and to come to Him so their souls may live.