In the days of the divided kingdom, faithless King Ahaz of Judah fearfully anticipated an invasion by Syria and Israel (Isa. 7:1-2). All of Judah trembled with him. Was Judah done for? But what about the Lord's covenant with King David and the promise that David's house and kingdom would be made sure forever (2 Sam. 7:16)?
Interestingly, the qualifier made sure comes from the Hebrew verb 'aman, (from which the word "amen" is derived), which means "to be reliable, to support." In other words, the House of David was made secure in the covenantal promise of the LORD.
In response to this crisis, the LORD sent the prophet Isaiah and his son to the king to proclaim the message of deliverance (Isa.7:3-9). The phrase im lo ta'aminu, ki lo tei'ameinu employs a play on the Hebrew verb 'aman. The House of David will indeed be established ('aman), but only through faith (emunah - the noun form of 'aman) in the promise of the LORD. That is, the House of David will be established only by faith.