Isaiah 26:3

You will keep in perfect and constant peace the one whose mind is steadfast (that is committed and focused on You-in both inclination and character), Because he trusts and takes refuge in You (with hope and confident expectation).

What we focus on and think about affects our state of mind.  When the Bible is at the core of our daily life, we can be guided by God and kept in peace.  If the world is buffeting us with all its temptations and distractions, then they become our focus we will lose our peace.  Let not your heart be troubled but keep God as the focus, and peace will prevail.