build on sand

From the Parable of the Wise and the Foolish Builders in Matthew 7:24–27.

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build on sand (third-person singular simple present builds on sand, present participle building on sand, simple past and past participle built on sand)

1. (idiomatic) To put something in an unstable position by failing to give it a secure foundation.

   Loveless marriages often end in divorce because they are built on sand.