Why used "firm anchor" but not "a firm anchor?"

Context:


(Episode from The Language of God)
In complete honesty, I am still working on that. Perhaps
this was also an opportunity for me to recognize that I could
not truly protect my daughters from all pain and suffering; I had
to learn to entrust them to God's loving care, knowing that this
provided not an immunization from evil, but a reassurance that
their suffering would not be in vain. Indeed, my daughter would
say that this experience provided her with the opportunity and
motivation to counsel and comfort others who have gone
through the same kind of assault.
This notion that God can work through adversity is not an
easy concept, and can find firm anchor only in a worldview
that embraces a spiritual perspective. The principle of growth
through suffering is, in fact, nearly universal in the world's great
faiths. The Four Noble Truths of the Buddha in the Deer Park ser-
mon, for example, begin with "Life is suffering." For the believer,
this realization can paradoxically be a source of great comfort.