Those who leave often come back and then we know they were once partakers of the faith; thus the principle of the Prodigal Son.
I agree, but this is not who John is referring to in 1 John, he's teaching about the gnostics who were infiltrating the church with false doctrine and leading others astray. John is writing about these teachers who were never genuine although they said they were (1 John 2:4), nor the ones who they took with them. We know this because they left (1 John 2:19).
If someone, not only these ancient gnostics, but false converts today, leave the faith (begin believing heresies) they were never believers to begin with. Not to say they cannot believe in the future, but the Bible gives no evidence of anyone who was a genuine Christian at one time, became a heretic for a time, and then became Christian again. I'm not talking about what we call "backsliders" today (although I'm not sure there is such a thing) - I'm talking about people who outwardly behaved and confessed Christ but never knew Him. Remember what Christ said of those on the Day who will seem bewildered when He says, "I never knew you"?
Why you keep bringing up the prodigal son is somewhat baffling to me because I see the two as apples and oranges. Besides, the prodigal gives no evidence of ever being right with his father, it's not a story about a person who had a relationship with God as Father, became distracted for a time and then was restored. This kid was the lowest of the low, he wanted his inheritance before his father had died which would have been like a complete slap in the face to his father, and the Jews hearing the story would have known that. The prodigal represents a sinner at his worse, squandering his life and all the blessings of God to live with the hogs. He only came home because he had hit rock bottom. The fact that the prodigal was willing to disgrace his father in that way showed he never loved him in the first place. I think the story represents the prodigal as one who, in spite of sinful and lascivious living, can still come to the Father and receive mercy and grace.