"Ask a sign of the LORD your God …” (v11). King Ahaz of Judah was terrified because his rival, the king of northern Israel, had allied with Syria against him (v1-4). God called the two threats “smoldering stumps of firebrands” (v4) and told Ahaz “Be careful, be quiet, do not fear” them (v4). God wanted to give Ahaz a sign of His power in the situation, but Ahaz was too timid to ask (v12-13). Finally, the sign given was of the future Messiah (v14).
“Do not let your heart be faint …” (v4). God wants us to ask Him boldly for the help and mercy we need. Since He has already won our victory through Christ, we have nothing to fear.