Through common grace Christians and non-Christians can work together for a common cause. Common grace does not save, but does preserve man from full expression of his own depravity (Jer. 17:9).
For example, I've known liberals who oppose abortion, and there's a gay/lesbian group called PLAGAL: Pro-Life Alliance of Gay and Lesbians. Even though I would take issue with their personal lives I would be loathe to reject their work for the protection of the unborn regardless of motives. The pro-life movement is not a monolithic, well-funded bloc but a rather rag-tag melange united by a common sense of justice.
Remember the prostitute Rahab and her works.