One way communication is not communication, FK.
What? Of course it can be either. When I send this post and you read it I have communicated to you. If you choose not to respond then I have still communicated to you. Does a communique cease to be a communique if it goes unanswered? Of course not. The response would be ANOTHER communique. And, BTW, the communication we have with God starting with prayer IS two-way because we are told that He answers our prayers, in some form. That is also communication. Of course He doesn't "pray back" to us, but He uses other methods to answer in communication. For example, we see the result of our prayer, one way or the other. That is communication.
FK: "So do you believe that there can be a true believer, who goes through his life never praying to God, and nevertheless go to Heaven?"
According to the Reformed theology yes, very much so. Prayer is not a coniditio sine qua non of salvation.
I also believe that perseverance is required. I can't imagine perseverance without prayer. By your fruits you shall be known. To me, under normal circumstances no prayer = no fruit.
A true believer should never ask for anything. If you trust God then you have no reason to. He provides for everyone in ways He sees fit. And if you are Reformed that should be particularly obvious and true because all that shall happen to you and the world has been determined and will happen exactly as God wants is, your prayers notwithstanding.
You are talking in theory and not in the real world. Many were called righteous and they were all still sinners. They also certainly asked for things. The Bible encourages true believers to come to God always with our petitions. So, we do. That's all I need to know. As I also said, this is the same logic that says Reformers shouldn't want to evangelize. Rubbish. It's not for us to decide whom is going to be saved, and it is not for us to decide which of our prayers will be answered favorably.
FK: "After the flash of light and voice from nowhere, Paul immediately says: "Who are you, Lord?" (Acts 9:5). That is a prayer."
So, every hallucination is a prayer? He had no idea what that was. How could he pray to something unknown?
Did Paul also hallucinate his blindness and conversion to follow Christ? The text tells us that he DID have an idea, and we are told what it was. We are not led to think that Paul thought it was a lightning bolt, or any other natural phenomena. He knew his idea of the Jewish God and obviously thought this was some manifestation of that God.
God knows what you want; there is no need for Him to hear it from you. You don't have to communicate to Him for Him to know that. This is like you telling me that you are married. I know that already! Communication should be meaningful and not stupid repetitions.
The most basic (dictionary) meaning of the word "to communicate" is to impart knowledge of; make known. My point is that we don't make anything known to God by praying; He already knows what we want or feel or intend.
And, BTW, the communication we have with God starting with prayer IS two-way because we are told that He answers our prayers, in some form. That is also communication
God's revelation to us is communication, as it imparts knowledge in us about God and His plan. But it is always a one-way process. He reveals to us what we don't know; we reveal nothing to Him He doesn't already know. So, our prayer is not communication, but supplication. We ask God for mercy, health, and so on. If you think about it, we do that because we are insecure. We want assurance, a sign. It comes from lack of faith. Our prayers should be nothing but giving thanks.
Did Paul also hallucinate his blindness and conversion to follow Christ?
Possibly. The account of that event exists in at least two different versions in the NT, so it is hard to tell which version is correct, or what exactly took place.