Abraham? Cornelius? Nathaniel? Jn 3:21?
Yeah I know, for some reason you're convinced it can't be more than a nanosecond.
The problem we have here is understanding what regeneration is. In relation to salvation the word means to be reborn or made new. You wonder why I'm convinced it can't be more than a nanosecond? Because the overwhelming evidence of scripture supports such a view.
Acts 10 starts off by introducing Cornelius, "a devout man and one who feared God with all his household" (Acts 10:2). Nowhere in the Cornelius narrative do we read that Cornelius was not already saved. Acts 10 teaches us that the Gospel was not be preached only to the circumcision, but also the Gentiles (vs. 34-35). In other words Cornelius, and those in his household, were being acquainted with message of Jesus Christ. Cornelius was saved much in the same way that Old Testament saints were saved; by faith in God according to the knowledge they possessed of the scriptures at the time.
In John 3:21 our Lord is not referring to regeneration. I understand why you would think it does. You believe those who come to the light only do so because they have been regenerated. In a larger context I simply see a contrast between those whose deeds are evil and those whose deeds are good. Eph. 2:10 states that Christians were created for good works. Jas. 2:18 says that some will show their faith by their works. In Mat. 5:16* Jesus states that we are to let our light shine before men so they may see our good works and glorify our God who is in heaven. These passages are not directed at those who have been regenerated but not justified.
Abraham was called to go to go possess a land and father a nation. We eventually read that he believed God and it was credited to him as righteousness (Gen. 15:6; Rom. 4:3). Are we to assume that Abraham was regenerated before he finally believed God? I just don't see that in the text. I do see that he believed God and he was made righteous. That is when his regeneration took place.
Jesus' proclamation to Nathanael in John 1 is bit more complex and has roots in the Old Testament understanding of Israel's purpose and her eventual corruption. But just at face value, here again we may have an individual who believed in God to the degree in which God was revealed through the Old Testament scriptures. I believe it is a stretch to read pre-justification regeneration into this passage.
Finally, I have no problem with you and I disagreeing over this issue. There are very good Reformed friends of mine that believe the way you do. We agree on so much more and have wonderful fellowship together. I write this so you don't think I am getting snarky with you. I have an obligation to my own conscience to state the truth as I see it. I would like to think that my fellow Christians share the same conviction and can extend charity to each other when we disagree, even if our debate can get somewhat vigorous.
*In Mat. 5:16 Jesus is addressing the nation of Israel. His comments are not directly solely to believers at this time, but to what Israel as a nation is supposed to do (c.f. Isa. 42:6; Luke 2:32).