1 John 2:19 - They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us.(YTL)
When the apostle John wrote his first epistle, it was likely after he'd written his gospel account, due to the epistle building on concepts and themes he had explained at length there. In this epistle, these things are taken as understood. Confirming this is the way a form of Gnosticism prevalent near the end of the first century is inferred in the epistle. According to these notes, we can see what had become plain:
"The Gnosticism addressed in the NT was an early form of the heresy, not the intricately developed teaching of the Cerinthian variety. This heresy was also libertine, throwing off all moral restraint.
Consequently, John wrote this letter with two basic purposes in mind: (1) to expose false teachers (2:26) and (2) to give believers assurance of salvation (5:13). In keeping with his intention to combat Gnostic teachers, John specifically struck at their total lack of morality (3:8-10)..." NIV Study Bible, 1987 Ed., p 1866
Now, with this background in mind, what does the text in question make plain? First the text is dealing with particular people. It is making something plain about specific people who had been involved with the first century Christian Church.
Is John making plain the original state of 'they' [these people] (you ask), or the [present] state of these people (is what I infer), or both of those things? John's language was plain enough for his readers. They knew who those people were, for those people had mixed with them but now had left them. In the previous verse, 18, John had identified them as being 'antichrist' (plural). He spoke of them as being 'many'.
"John assumed his readers knew that a great enemy of God and his people will arise before Christ's return... But prior to him will be many antichrists... The antichrists referred to in John's letter were the early Gnostics. The 'anti' in antichrist means 'against' [or, 'in the place of'.]" (Ibid. p 1869)
John does not say anything about the origins of Gnosticism. He does not show the origins of it. He simply takes the fact of Gnostics having come into the Church by the time he wrote that warning, but of having then left the Church: "They went out from us, but they did not really belong to us." (NIV) That is what had become plain.