Believers have indeed experienced life from death already. Not physical life from physical death as you seem to be implying. But spiritual life from spiritual death for those who "were dead in trespasses and sins." (Eph 2)
Where in scripture does it distinguish between physical death and spiritual death, as you are? Ephesians 2 does not do that. Paul describes the sinner as "dead in sin", but he's not explaining exactly what he means by that, and he's certainly not distinguishing between spiritual death and physical death. I see how you're deducing your doctrine from this, but that is all it is, a deduction. With that in mind, you have to consider other possible deductions.
My understanding from Ephesians 2 is that in sin, there is no eternal life, but only the promise of death. We see that elsewhere in scripture as well, that "the wages of sin is death". I believe this is what Paul was referring to, which is very different from what you're saying.
The Scripture calls this having part in the "first resurrection" and how we overcome the second death (Rev 20).
I don't see scripture saying that anywhere. That is your understanding, but it does not explicitly say that, so therefore you have to allow that your understanding could be wrong.