“the head of Ephraim is Samaria.” The capital city of “Ephraim” (Israel), is Samaria (see commentary on Isa. 7:2). And the king in Samaria was Pekah, the son of Remaliah.
“If you are not firm in your trust, you will not be firm in life.” The Hebrew text uses a word-play. The verb “firm” is repeated twice, the first time in the hiphel tense and thus having an active sense, the second time in the niphel tense and thus having a passive sense. The actual Hebrew text is short and would be literally translated as something like, “If you are not firm you will not be firm.” The NET gets the idea using the word “remain”: “If your faith does not remain firm, then you will not remain secure.” The NIV uses the word “stand”: “If you do not stand firm in your faith, you will not stand at all.” Many versions ignore the word-play and try to get the sense differently. Thus, the NASB has, “If you will not believe, you surely shall not last.”
This sentence is short but powerful. The great key to success in this life and in the next is trust in God and obedience to Him. Even with all the evil Ahaz had done, his kingdom would be spared and would be prosperous if he would trust and obey God. As it was, his kingdom escaped total destruction but suffered attacks in which his people suffered.