The action completed with enduring effect is still conditioned--IF you continue. What happens if you don't continue? Your "righteousness of faith" is forgotten (Ez 3:20)--that "history" retroactively never occurred.
I understand how you are looking at this, however, I have to insist, that's not how the perfect tense in Koine Greek works. And that's not the form of the statement. All I can do is repeat these same things. But that's the language. That's how it works.
What if you don't continue? It means you didn't pass the condition, and therefore, you had not become a partaker of Christ. That's what the passage says. That's the use of the first class conditional clause, This is true of you, IF that is true of you. You've become partakers of Christ IF you continue to the end. If you do not continue to the end, you had not become a partaker of Christ.
At then end of this part, you assert that the completed and enduring action of a perfect tense verb isn't completed and enduring.
Let me ask you . . . when God "forgets" a man's righteousness, are you thinking that means God becomes unaware of that man's history? That his history ceases to exist, and God Himself doesn't know what his history was? I don't see it that way.
But this goes back to the difference between the covenant God made with Israel, and the covenant God makes with us. God's covenant with Israel was based on the people's righteousness as determined by the Law. God's covenant with me is based on Jesus' righteousness as determined by His life and death and resurrection. The people's righteousness comes and goes, as we've seen, but Jesus' righteousness remains forever.
1 Peter 1:22-23 KJV
22) Seeing ye have purified your souls in obeying the truth through the Spirit unto unfeigned love of the brethren, see that ye love one another with a pure heart fervently:
23) Being born again, not of corruptible seed, but of incorruptible, by the word of God, which liveth and abideth for ever.
Much love!