A brief anecdote will help illustrate what I mean. I can remember when I first started playing bass guitar in the band. I borrowed a bass that, naturally, already had strings on it. I played it for close to a year. Then, I bought a brand new bass with brand new strings. There was no need, in either case, to change the strings. I had already learned a dozen songs and played a good number of concerts before the inevitable happened--I broke a string. At that point, it would have been totally reasonable for you to assume that I knew how to change the strings. I was in a band. I had played shows. However, you would have been totally wrong. I can still remember asking the other guys in the band to teach me how to do it (yes, this was before YouTube). It is important to note that they probably would not have ever gone out of their way to ask me, since they also would have assumed that I knew how to do it. In the same way, people aren't going to wave a flag around that says "I don't know this or that," about Christianity, but they still need to learn. We should not simply assume that because people go to church or call themselves believers, that they understand the basic teachings of Christianity.
There is already a tremendous number of people outside of the Church who have a very distorted view of Christianity. We only compound that problem by wrongly assuming that all of the people inside of the Church have a correct one. So what am I saying? I am saying that we need to start being intentional about discipleship. We cannot treat teaching the Bible and teaching doctrine like we might approach a wearing a suit to church--as a merely cultural element, something that they did back then, but now we have moved beyond. And I am not just talking about with young people, new Christians, or in a "New Members Class." At this point, there are a lot of people who have been in church for decades, people who are kind and sincere, lead sparkling lives, but really don't know anything other than a few popular worship songs and slogans like "God loves you and has a wonderful plan for your life." When it comes to the basic teachings of Christianity, we cannot make "the Geico mistake" and assume that "everybody knows that."