I don't think you have a good grasp of Protestantism if you think that they just throw out the Old Testament.
which means they start by asserting that the Holy Spirit is imperfect because the Holy Spirit cannot and did not protect Scripture from the inclusion of error from prior to Christ until the 1500s.
What? That's absurd.
In fact there's trinitarin themes in the Old Testement, kinda buried / not-as-obvious as the crucifixion-passover foreshadowing.
Here's Dr. Michael Heiser's presentation/lecture of The Jewish Trinity: How the Old Testament Reveals the Christian Godhead which is looking at the Old Testament alone specifically with the idea of [answering] "can we look at only the Old Testament and get the idea of the Christian Godhead?"
Of course it’s absurd but the truth never stopped the Prot haters from bashing Protestants..
The Anti-Christ, Anti-Christian, Pharisee Approved, Luther Subset of the Old Testament, is not the Old Testament.
The Septuagint preexisted the incarnation of Jesus Christ.
Jesus Christ took the Sadducee and Pharisees to task for adding to the law and many other things.
If they had added uninspired Scripture to the Word of God how can anyone believe Jesus Christ wouldn't have pointed that out ? Especially anyone who claims to base everything they believe on Scripture Alone?
No Apostle ever said there was anything in the Septuagint that shouldn't be there and the Bereans were praised for searching the Scriptures, Scriptures that for the Greek Bereans were without a doubt the Greek Septuagint. It's laughable to claim anti-Christ, anti-Christian, Pharisees who survived the destruction of Jerusalem and therefore could not possibly have had any legitimate authority from the then dead Jewish priesthood have more authority than Jesus Christ. The only authority this theoretical group had was whatever they granted their themselves.
If you obey that group of Self-Appointed anti-Christ, anti-Christian, Pharisees who had no connection whatsoever to the legitimate Jewish authorities with regard to the Old Testament rather than accepting the fact that neither Jesus Christ or the Apostles ever said a thing about there being uninspired Scriptures in the Septuagint, then you are by definition a Pharisee, not a Christian.
If you believe your fellow anti-Christ, anti-Christian, Pharisees are correct and that the Bible as translated into many, many, languages and distributed throughout the world up until the time of Luther contained error, then you have to believe not only that Christ and the Apostles didn't care that the Old Testament contained uninspired books, but also that the Holy Spirit cannot and did not protect the Word of God from the inclusion of error for roughly eighteen hundred years.
If you believe that, so be it. What logically follows is that the Holy Spirit is imperfect and therefore cannot be a part of the Trinity. Since you base that denial of the perfection of the Holy Spirit on your own personal superior to the Holy Spirit Self and Self Alone acceptance of what anti-Christ Pharisees may have said, you elevate your Self into the Trinity as coequal with God the Father and God the Son replacing the Holy Spirit with the Holy Self.
Hence, you worship the Trinity of God the Father, God the Son, and God the Self.
Do as you see fit but you when you hear Jesus Christ say, "I never knew you", you won't be able to plead ignorance since neither Christ or the Apostles ever told anyone to throw out a portion of His Holy Word.
True; but it took you off your game, to go into defensive mode, and waste time posting information that they couldn't care less about.