RE: “Just how much you are suffering under the general unrest and distress, in whatever manner, depends largely upon how much your soul is rested in God.”

It is altogether unhelpful to tell a deeply anxious person that he will find relief if only he has enough faith. If he actually believes you, then your remarks may very well induce feelings of guilt and further anxiety. It would be much more helpful, I should think, to actually exercise faith and pray together, "casting all your anxieties on him, because he cares for you" (1 Peter 5:7 ESV).

A dear Christian friend of mine is being treated for anxiety and depression. Because of what I know about how her difficulties arose, I refuse to believe that her suffering persists because of a deficiency in her faith.

”For [God] says to Moses, ‘I will have mercy on whom I have mercy, and I will have compassion on whom I have compassion.’ So then it depends not on human will or exertion, but on God, who has mercy” (Romans 9:15-16 ESV).

RE: "But if their heart is fixed in God, they have peace within under all circumstances..."

Note: And [Jesus] took with him Peter and James and John, and began to be greatly distressed and troubled” (Mark 14:33 ESV).