The Bible will not answer all our questions, and that is never more obvious than when looking at the first few chapters of Genesis. The Bible gives us a huge amount of information; some people set ambitious goals of reading through the entire Bible in one year. Many fail while others have never even tried. Our goal is to see what the Bible is saying to us and trust that it is enough. We have what God means for us to have.
It is still hard to read a selection such as Genesis 4 and wonder “Who told they guys to build altars? Why are they sacrificing anything?” We know there were occasions that God spoke to human beings. Presumably everything he said did not get written down. He could have easily communicated instructions that are not recorded. We have a certain understanding of what the historical record should be based on Western Civilization from around the time of Philip and his son Alexander, circa 300 B.C. or so. That is the beginning of the modern era; ancient historians, of Eastern civilizations at that, did not think about creating a complete historical record the way we do.
Let’s look at what scripture tells us of Cain and Abel. The ancient text may not stand up to our modern literary criticism but there is nothing we can do about that.
A friend in-real-life was listening to one of my sermons recently and had a question. He wanted to make it clear that I was not going over his head but wondered if everyone in my congregation was always able to follow. I told him that some of points in the case I was making were repeated from things we had either studied or I had preached before. I kind of figured they would remember some of it. The other thing I pointed out was that my particular audience had a lifetime of experience; some of those church members had been at that church since before I was born. I would tailor my presentation for a youth group or a congregation with many new believers. You gotta know your audience.
The Master's Table 