The last sentence is a play on two Hebrew words: If you do not stand firm (te’amenu) in your faith (ta’aminu), you will not stand at all. Firm trust in the Lord is essential, especially for a leader of God's people (see also 2 Chronicles 20:20). Leaders must act on their faith to show it exists.
Ahaz and the people of his time trusted Assyria, their enemy, instead of trusting God. They looked for help from a foreign power rather than from the Lord.
Later, King Hezekiah faced a similar threat. In that situation, he showed faith in the Lord (see Isaiah 36–38).