You know what happens when we assume? We end up having a church for insiders, and a church with little impact.
Vince Antonucci, Lead Pastor at Verve Church in Vegas says "your assumptions will determine your crowd". What he's saying is that you need to not assume base levels of knowledge in your church experiences or you will, unwittingly, exclude people who don't have that base level of knowledge.
That's why we ALWAYS explain where things are in the Bible, it's why we don't say "you all know the story of . . .", because many people don't. AND, if we talked that way, it would be easy for that person to feel like they are an outsider and don't belong. And the more you talk with assumptions, the more of those people walk out and never come back, and before you know it, you have defined your congregation by those assumptions.
If you're a church leader, or church planter - this is such a HUGE issue. Don't dismiss it. You can say you're going to create a church that unchurched people will want to come to, but if you let your assumptions get in the way, you will NEVER have that kind of church that you dream of.