From Isaiah

This is what I was reading today, and the questions that struck me as important as I read it. I'll let you answer them.

Isaiah 26:3-4

The steadfast of mind You will keep in perfect peace,
Because he trusts in You.
Trust in the Lord forever,
For in God the Lord, we have an everlasting Rock.

Questions.....

Who is the promise for?
How long do we need to trust?
Why should we trust God?
Finally....why is this important for any of us?

(Hint: The first three are answered in the verses. The last question is for you to think about, but note the assumption in the question.)