What is the 'missing' link between Islam and Christianity?

We know that Islam and Christianity somewhat have something in common,or at least,basically,serve the same God but they just do it in different way. Now there’s a question about the separation of both of this religion,Christianity could have diverged from Islam and vice versa but of course,there is this missing link,probably a minor religion that connect the relation between these two major religions. My guess that is Jew but I could be wrong. What do you think?