build (something) on sand

To create, provide, or use an unstable or impermanent foundation for something, such as a building, business, or relationship. It is taken from a parable in the Bible (Matthew 7:24–27 and Luke 6:46–49) in which Jesus warns that those who do not follow Him build their lives as houses on sand, liable to be washed away by the elements. In hindsight, I guess our relationship was kind of built on sand, since we didn't really have much in common to begin with. Without proper financing, you'll end up building your business on sand!