I’ve never seen them as opposites, though I can see how they conflict. In my mind, existentialists believe that there is no life purpose other than what an individual fashions for him/herself. But I don’t think an existentialist would deny the possibility that an individual’s desire to be, say, a circus clown, can stem from an external factor (like being raised in a family full of clowns). They would just argue that the individual shouldn’t believe that being a clown is his life’s purpose simply because that’s what his ancestors have always been. He has the freedom to define his own purpose. He may decide his purpose is to serve God every second of the day while wearing that clown suit. He may decide his purpose is to seek physical pleasure every second of the day. He may decide he doesn’t have a purpose at all. He has the freedom to rationalize his life however he wants, since there is no universal or absolute purpose to our existence anyway.
The purpose he assigns his life is probably going to be quite predictable (a person raised in a Christian family has a high likelihood of believing he has a Christ-centered purpose). But it’s still a purpose that’s totally self-imposed. The person who rejects the Christian faith hasn’t rejected his life purpose. He’s just found something else, perhaps.