Mathew 8:17

That is might be fulfilled, which was spoken by Esaias the prophet, saying, Himself took our infirmities and bare our sicknesses.

The more we see the truth of the word of God, the less the devil can defeat us.  The burdens of this life so easily slip in, and we pick up some thought that the devil shoots in our direction to get us to doubt.  Then some symptoms or a difficultly and an obstacle than come out of nowhere, and we doubt.  The word remains the same firm and true to our lives.  Our eyes sometimes need to refocus on that truth and shut the world out.