Some Christians - nominal believers and real believers - don't get that they can't compartmentalize their faith into a my-religious-life compartment and a the-rest-of-my-life (i.e., my-real-life) compartment. Having different rules for different compartments of life is a recipe for incongruity and hypocrisy. Many Christians latch onto a part (or perceived part) of Christianity and ride that hobby horse to the neglect of the whole of the Christian faith. For real believers, it's what I call error-by-emphasis; for nominal believers, it's Cafeteria Christianity, picking what one likes (real aspects of Christian teaching or baptized personal views) and leaving behind the rest (things like sin, redemption and personal need for salvation are almost always left behind).
An example of error-by-emphasis that I think is pervasive among Evangelical Christians is a fairly tight focus on the basics of sin, redemption and salvation. It's not an error-in-fact; what is taught is true. The problem lies in what is not taught or given little emphasis. Fundamental things, like what God says is right and wrong, what God says is justice - essentials in practical everyday life - are much less taught, leaving believers to do what they think to be right.
This latter problem was brought home to me a while ago when I realized that I had failed a Christian friend. He was a new Christian and was in a serious dating relationship. He moved in with her. I assumed he realized that this was not right, until, some months later he mentioned that he had been reading his Bible and had come to realize that what he was doing was contrary to what God said was right. Soon after, they realized that she was pregnant, and they got married (there's much that I'm leaving out for the sake of brevity). I should have known better, or at least known enough to say something, but I really did not realize that he had never learned what God had said about sex outside of marriage. The Ten Commandments (in age-appropriate detail) had been part of my upbringing from elementary school and for some reason I didn't imagine others' upbringing being that much different.
A time came a year or two later when I was in a similar situation with a Christian friend who was taking his relationship with a girlfriend too far. That time I did say something. It still didn't go well, but I had the satisfaction – very small, because of how it all turned out – of not having failed my friend.
In reflecting on my church experiences up to that time and the decades since, I've realized that basic foundational things like what God says is right and wrong receive insufficient attention in Evangelical circles. Teaching the basics of salvation is necessary, it's truth. The problem isn't untruth but imbalance.