When I see different Christians aligning themselves with unbelievers for a good cause, I am concerned that the unbeliever will think of themselves as a Christian and be misled. Should a non-believer and a Christian be standing together in this way even for a good cause?
Francis Schaeffer used the term co-belligerents to explain Christians working beside non-Christians. This is a good article that explains how it can be viewed. It might help somewhat, I'm not sure. But I do agree we must never dilute the gospel or make people feel that they are Christ-followers if they are not; that is not only untruthful but unkind, because it keeps them from Christ and the salvation they need.