Notice how many times the phrase "in him" exists in this section. As Paul is dealing with the influence of those who are trying to impress upon these Colossians the righteousness of works via ceremonies, festivals, new moons, sabbaths, etc..., and by such are being bound as with a halter, from which Christ has set them free, he reminds them of their unitedness to Christ and of the forgiveness and imputed righteousness that they were united to 2,000 years ago that sets them free from this garbage. Their reconciliation with Christ was a past tense event that occurred before they were born, as he was their head and representative.
So, Ch.2,vs.6 tells them to remain in Christ (walk in him), be rooted and built up in him. Vs.9, in him the whole fulness of deity dwells. Vs.10, you have already been filled in him. Vs. 11-12, in him you have the covenant sign that you are his; you were buried with him in baptism, raised with him. And, vs.13, God made you alive together with him, having forgiven us all of our trespasses.
I don't think he's talking about regeneration here, as much as he's talking about them being united to Christ in his death and ressurection and of being made alive when Christ was made alive. As Christ is the head and representative of all of his people, when he was quickened, they were quickened with him. When he rose from the dead, they rose with him. This "having forgiven you all trespasses" was a past act, as it was done and over when Christ died. When our sins were imputed to Christ 2,000 years ago, and he shed his blood for them, forgiveness of sins was done and complete. This cancelling of their record in vs.14 happened at that time, as well as the disarming of the rulers in vs.15. Therefore, vs.16, he gets to his point, to "let no one pass judgment on you in regard to these forms of works righteousness. Your righteousness and forgiveness is already a settled issue, as it flows from you being united to a historical event that occurred before you were born. Vs.20, again, "if with Christ you died" (2,000 years ago); and Ch.3, vs.1, "if you have been raised with Christ (2,000 years ago when Christ was raised).
He seems to be speaking the entire time of us being united to Christ in his actions of death and resurrection. He being our head represented us during that time. God united us with Christ, so that when he rose, we rose to life; and that seems to me to be what he's referring to in Ch.2, vs.13 and in most of the surrounding letter. I don't think he's speaking at all about our space/time experience of it, in which we were born again and actually were regenerated to life.
Much of this thought came from Gill's commentary on the verse. You can find his comentaries on this website:
John Gill's Exposition of the whole Bible.
Blessings!