The essence of a continuous integration is that the system is regression tested all the time people develop and commit new features or bug fixes. The basic assumption is that the code is always tested for regression on multiple levels; unit, component and system/integration level.
This leads to an interesting question: Can you really have a functional continuous integration system if you don’t drive your development with tests?
The simple answer is no.
It is certain that no-one will write comprehensive unit tests after the code has been put to production. And without unit-level automated tests you can not have fully functional CI environment. If you don’t drive your development with tests you are creating technical debt.
This does not mean that you have to become a TDD zealot, but it means that you should start to develop with TDD or a variant in order to achieve fully functional continuous integration.