Faith brings peace and certainty when we trust the Lord (Isaiah 26:3-4)

Isaiah 26:3-4 (NIV)
You will keep in perfect peace
    those whose minds are steadfast,
    because they trust in you.
Trust in the Lord forever,
    for the Lord, the Lord himself, is the Rock eternal.

To know God in this way it help if we know ourselves in view of God's design (Psalm 139)