Half Banana...
I understand that to be the general context of bible writings. Wether you believe or not, analyzing the book, in my opinion, shows that God had to show himself to the world and prove himself as well. He could not expect people to know him without evidence. It was basically a response to the questions posed above (how do you get to know an invisible spirit). You can't. The invisible spirit would have to make himself known to you and then you may desire to know more and continue to nurture the relationship further. You can not love what you don't know.
The bible has several passages in which people (Abraham was one of them), meet this God for what it seems the first time. True, the bible's first account of God talking to Abraham does not provide details of the communication. But later on, Abraham sees a man, whom he recognizes as God. Never the less, every other encounter like these from different biblical characters, are accompanied by some kind of "prove me that you are God and you are capable of delivering your promise". Some miracle that convinced men this God was who he claimed to be. Very much the same way you prove your credit worthiness by keeping your debt in check, God kept his worthiness by showing his followers what he was capable of, time and time again. This way he showed himself to the world and showed his followers he could very much fit the job of a God who claims to have created the universe.
Having been absent from the human race for thousands of years (correct me if I am wrong but I don't think he's ever been absent for so long before) and having many generations of humans pass without testimony of his fame, God would have to present himself again and make himself known if he plans to have any followers for much longer.