That verse does not prove what you think it does DHK. It doesn't say:
"Therefore, having been justified by faith (once in the past) we forever have peace with God through our Lord Jesus Christ,"
But we do forever stand before Christ, having been justified and having access before God. I will always have peace with God, but there are times that I may not have the "peace of God", and that is when sin may cause my fellowship with him, my walk with him to be strained. But I will always have peace with God. I will never again be his enemy.
"Forever" is word you assume is implied there but is not. In fact you yourself wrote: "...we have peace (PRESENT TENSE) with God" (which of course is not necessarily FUTURE tense), and that part is true as long as one considers the NEXT verse:
"through whom (Jesus) we have (present tense) access by faith into this grace in which we stand (present tense)" (v2).
It is absolutely true, just as it is written. I have access by faith into this grace in which we stand. My standing before God is possible because of my justification. I am counted as holy before Him, justified, regenerated, born again, converted. My sins (past, present and future) have been atoned for and completely forgiven. And herein do I stand.
Hebrews 4:15-16 For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin.
16 Let us therefore come boldly unto the throne of grace, that we may obtain mercy, and find grace to help in time of need.
--My standing before God is as a priest. I now can come before God directly, right before his throne of grace. No other person except the high priest in the OT could do that. I stand in the righteousness of Christ and am able to stand before Christ, before his throne and offer my petitions before him. I am made righteous by the blood of Christ.
You see, having present tense "peace with God" is contingent on one's present tense standing in God's grace through the access of present tense faith.
The faith I put in Christ (concerning salvation) was a one time act.
The resultant peace I have now and have had ever since I have been saved. That you cannot contradict because it is true. The Holy Spirit came and by the power of Christ dwelt in me, changed me, gave me peace and joy, and that has remained ever since I have been saved. He is still changing me every day to continue to be more conformed to His image, less conformed to the world (Romans 12:2).
My present standing with Christ will never change. I am justified with Christ by faith. It is and always be that way.
And, Paul is clear in the same epistle that it is possible that those who currently "stand (present tense) by faith" (ie the Gentile believers in Romans 11:20) can be "cut off" if they do not continue in the goodness of God (11:22).
Why go there? You clearly take Scripture out of context. Romans 9-11 are speaking of the nation of Israel and speak nothing of our salvation. We have hashed this out over and over again. You cannot take Scripture that speaks of the nation of Israel and force it to a different context such as Romans 5. It doesn't work.
Paul is also clear to the Corinthian believers who are (present tense) standing in the gospel, that they will have "believed in vain" if they don't hold fast that same gospel by which they are saved (1 Cor 15:1-2).
Again CONTEXT!!! What is Paul saying here and what is the context? The context is the resurrection. Look up the chapter. He is saying if there is no resurrection then our faith is in vain. The chapter is introduced by giving a summary of the gospel. And he tells them that they did not receive the gospel in vain.
What if they did?
1 Corinthians 15:19 If in this life only we have hope in Christ, we are of all men most miserable.
--But they did not.
Perhaps there were some false teachers among them that were denying the resurrection and thus this chapter was written. It is the greatest apologetic for the resurrection ever written.
Also, in his second epistle to the same church he warns them not to receive the grace of God in vain (2 Cor 6:1) implying it is possible to do such a thing.
Read the chapter. There were false teachers among them. He was warning them not to listen them, not to be persuaded by them. Look farther down the chapter:
2 Corinthians 6:17 Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing; and I will receive you,
--Come out from among them (the false teachers).
Don't be persuaded from them.
This had nothing to do with losing their salvation.
So if one doesn't continue to have faith, he no longer continues to have access into God's grace, and thus no longer has peace with God through Christ, and thus as no ongoing justification before God. Try as you might, you can't make Paul teach a once-for-all, irrevocable justification irrespective of the presence or absence of actual ongoing faith.
Nowhere does it indicate that one must continue in "saving faith."
Nowhere does it indicate that a person cannot have a standing before God, unless he is unsaved.
Nowhere does it indicate that a person once justified can lose that status.
If one is justified, it is a one time act never to be undone.