build on sand

Use an unstable foundation, as in If you buy nothing but high-risk stocks, your portfolio will be built on sand. This metaphor appears in the New Testament, where Jesus likens those who do not heed his sayings to a foolish man who builds his house on sand, which then is washed away by rain, flood, and wind (Matthew 7:24–27). [c. 1600]

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.