God communicated it perfectly, to those who care to know, and are willing to listen.
Do you know what salvation is, apart from reading the Bible? No, you don't.
In fact, most words used in the Bible can only be defined by the Bible.
If that were true, then
Adam would not have understood God
Noah would not have understood God
Abraham would not have understood God
The Bible, once it was written, wasn’t written in Hebrew and Greek, according to your position.
Rather, we know God spoke to people in the language they understood—even before the Bible was written. Thus, when they rejected His message, it was their fault, not God’s because He didn’t know how to communicate with them.