But see, that's the thing! Faith doesn't need the softening of waters of doubt. They are separate issues. Apples/oranges, whatever. We walk by faith.
It doesn't mean that in our walk of faith our carnal weakness trips on a rock of doubt. I know I can fully trust God. In a moment of weakness, I might try to make something happen out of fear.
Like Abraham and Issac we may falter at moments in our walk of faith. But faith still means the assurance of things hoped for, the conviction of things not seen. It doesn't need softening or hardening. It stands alone. It's what we have when we come to know Jesus.
This quote is like saying truth is softened by waters of falsehoods or it's not really truth. Who in the world are you quoting?
"Faith" isn't just 'God stuff.' Most of us already have plenty of misplaced faith. Abe actually had faith that he would be killed if he didn't take some deceptive action (ie: Having his wife lie.)