Worry is not a trivial sin, because it strikes a blow both at God's love and at God's integrity. Worry declares our heavenly Father to be untrustworthy in His Word and His promises. To avow belief in the inerrancy of Scripture and in the next moment to express worry is to speak out of both sides of our mouths. Worry shows that we are mastered by our circumstances and by our own finite perspectives and understanding rather than by God's Word. Worry is therefore not only debilitating and destructive but maligns and impugns God.
John MacArthur