The quote comes from Let Us Have Faith, a book published in 1940, i.e. after the beginning of World War II. (Keller mentions "the wrecking of half a civilization" in the book's first sentence.) Because of those dark times, Keller wanted people to have faith (page 1):
For faith is thought directed toward good, and like all thought-power it is infinite.
The quote in the question comes from the chapter entitled "Faith Fears Not", more specifically a paragraph that discusses "security". By security, I assume Keller means the concept of eternal security, also known as "once saved, always saved". Keller writes (pages 50-51):
Security is mostly a superstition. It does not exist
in nature, nor do the children of men as a whole
experience it. God Himself is not secure, having
given man dominion over His works! Avoiding
danger is no safer in the long run than outright
exposure. The fearful are caught as often as the
bold. Faith alone defends.
Calvinism affirms the doctrine of eternal security, whereas Catholic theology does not. However, Helen Keller's faith was influenced by the Swedish theologian Emanuel Swedenborg, who rejected the concept of salvation through faith-alone, a doctrine that was important to Martin Luther, John Calvin, Ulrich Zwingli and other Protestant reformers. This may have influenced Keller's stance against the doctrine of eternal security.