We primarily have three phases in out lives. We start with listening to our parents and live a life to statisfy their asks, next phase is when we start living life for our desires and try to satisfy our needs, the next phase in life is when we start living life which God wants of us and live life to satisfy His asks.
These phases vary with individuals some people live through all the three phases in the sequence, some people live only one or two of these phases. The important point here is some people reach the stage three at a very early age, some people reach that gradually and some people reach that in a very late stage of life. It is God who calls people to live that life.
Looking into the Bible, early in Genesis God told Adam, "do not eat the fruit from the tree of wisdom", next in Exodus God gave His people "The Ten Commadments", next in the Gospel God gave people "John 3:16".
The words what God said Adam "do not eat the fruit from the tree of wisdom", is not easily understandable. At that time Adam was primarily taking care of the Garden of Eden. Adam was following what God told him blindly for a while, but then serpant played its trick to make him eat the fruit. Now that is what happens to people who have blind faith with no understanding, satan can trap them unknowingly. When people ask me, why did God allow this to happen, why He did not stop it. The answer I say to them is that it is the love of a Father who allowed his son to go out and understand what is around in the world.
Now when the Israelites started to live their own lives, the Father God gave them "The Ten commandments". These are to protect their souls from being lost and destroyed in Hell. They were not called with the purpose to serve God or to spread the Gospel. They were only working to earn their food, to protect their family, have wife and kids. There were individuals like Moses, Joshua, King David, Samuel who were leading people to the God. But the faith was so restricted to the closed group of people.
Finally after the exile, when people wanted to return back to God, God gave them the key to salvation "For God so loved the world that He gave His one and only Son, that whoever believes in Him shall not perish but have eternal life". To understand what someone is saying, first you should understand that person. Else, it is difficult to interpret what He is saying. To know what God is saying we have to understand who He is. It has taken millions of years for Man to be able to understand who God is and to understand what is the purpose for which he has been created. Jesus Christ says that He calls us friends, because I tell you what my Father is telling me. The Holy Spirit will reveal to you what He is hearing from me and Father. Because knowing His is knowing the Father, and when we know the Father, reveals the understanding of what He is saying.