So if I am understanding this, you are giving causative force to the Greek perfect passive verb, is that it? I was unaware that the perfect passive had causative force. Please find a grammarian who agrees with you.Still, I have not articulated what I am thinking about the substantive participle. I don't think it is suggesting how the ones believing became believing, specifically. I think the perfect passive verb generally suggests why they are believing.
Let me employ this example: Let's replace the substantive participle with the generic "Christians." After all, regardless of one's theological persuasion, we all agree that Christians do believe in order to be saved and they are "believers." Doing so, we can restate the sentence this way:
"Christians have been born of God."
Of course, as most of the commentaries say, it is important what "the believing ones" are believing in--that Jesus is the Christ. I think "Christians" encapsulates that, at least for this example.
Now, taking the generic "Christians" in combination with the perfect passive verb (eliminating the discussion about the participle) it would be understood that the subject, "Christians," are that way because of God's past work, would it not?
This is what I'm getting at. The passive nature of the verb is, I think, the most important aspect to this verse. It shows God's work in the past--lasting into the present. The plain reading, then should understand that "the ones believing" (a simple fact) are believing due to God's birthing action in the past.
Blessings,
The Archangel
Consider the following sentence: "The running man was shoved into the light pole and injured." We here have in the English a similar sentence grammatically to 1 John 5:1, complete with a gerund modifying the subject, and a past passive for the verb. By your reasoning, the reason the man was running is that he was shoved into a light pole. And the person who shoved him into a light pole was causing him to run. Do you see my point?