If you read around the verse, you find this is God instructing Isaiah to tell Ahaz just these words. Notice it is first, be careful. Not be strong, be brave or be resilient, but to be careful. Take care to be calm and not be afraid. Being careful takes a self check of mindfully doing this... taking careful steps.
Isaiah 7:9 finishes it up with "If you do not stand firm in your faith, you will not stand at all." That took my breath away. Deeply touching me, these words strengthened me to know to live I must have a strong relationship with God. Anything can happen around me, and if I am one with Him, I will stand strong in my faith to endure and walk through what I must in this lifetime.
I love it when Lady L is in the room and voices around her begin to change in pitch and she immediately is coming to you saying, "Be Calm". Our of the mouth of the child comes the word of the Lord... Be calm in Him.
Words of my heart, Laurie
A Texas Artist Laurie Pace
In God, whose word I praise— in God I trust and am not afraid. What can mere mortals do to me? Psalm 56:4