Thanks for clarifying the time scale, i.e. up to the late 1800’s, and for attaching that link which explains what Lutheran’s used to believe. I say “used to believe” because it seems that modifications have been made to Lutheran beliefs that the Pope is the Antichrist:
The Wisconsin Evangelical Lutheran Synod - Lutheran refers to those who adhere to the teachings of Martin Luther, the leader of the Reformation in Germany. “In the 1860s, however, doubts about this confessional statement were raised within Lutheranism.... Since 1930 the ALC taught that it is only a “historical judgment” that the Papacy is the Antichrist... In 1951, the Report of the Advisory Committee on Doctrine and Practice of the Lutheran Church—Missouri Synod stated: “Scripture does not teach that the Pope is the Antichrist. It teaches that there will be an Antichrist (prophecy). We identify the Antichrist as the Papacy. This is an historical judgment based on Scripture. The early Christians could not have identified the Antichrist as we do. If there were a clearly expressed teaching of Scripture, they must have been able to do so. Therefore the quotation from Lehre und Wehre [in 1904 by Dr. Stoeckhardt which identifies the Papacy as Antichrist] goes too far.” This view was endorsed by the Lutheran Church—Missouri Synod Convention in Houston in 1953. https://wels.net/about-wels/what-we-believe/doctrinal-statements/antichrist/
While it is true that Luther considered the Pope to be the Antichrist, this is not central to Protestant theology. I doubt that many Protestants know much about end-time matters or that they have heard any sermons preached on the coming of the Antichrist, the false prophet, the two beasts of Revelation, and the destruction of Babylon the Great. During the 25 years since I became a Christian I have never heard any Baptist, Methodist or Free Church of Scotland minister preach on the Antichrist, let alone claim the Pope is the Antichrist.
Changes began to emerge with Roman Catholic counter-reformation propaganda which placed everything in Revelation into the future. Yes, it was the predominant view within Protestantism that the Pope is/was the Antichrist, but that is by no means the only interpretation of Revelation:
Historicist Interpretation: The reformers had identified the Beast with the papacy and its political influences over worldly powers. At first Rome and its satellite European Community countered this view with the Contemporary Interpretation.
Contemporary (Praeterist) Interpretation: Everything in Revelation was fulfilled in past history during the first century of the church.
Futurist Interpretation: Developed as Roman Catholic counter-reformation propaganda to place everything in the future. In chapter 4 of Revelation a secret rapture of the church is proposed. It is presumed this rapture will happen either before or mid-way through a future seven year period of tribulation. The thousand years is literal, the Old Testament priesthood will be restored complete with animal sacrifices and a new Temple in Jerusalem, with Christ Jesus reigning on earth and the church suspended in heaven above it. This scheme originated with J.N. Darby and other early Brethren leaders. The Plymouth Brethren adopted and propagated this originally papist subterfuge from the 1830's onward.
Resumptive Interpretation: Revelation applies to all ages of the church, unfolding the visible powers that lie behind the outward appearance of history. The invisible principles and powers are depicted with graphic imagery in which two opposing forces range over the heavens and the earth. Revelation repeats the period from the ascension till the judgment (when Christ Jesus returns to destroy God’s enemies). Numbers convey ideas, not arithmetic. Forms display concepts, not shapes. Words are clothed in mystery, not logic. Revelation is a vision, it is about spiritual light. Source: Introduction to The Revelation of Jesus Christ by John Metcalfe, published 1998
Some Protestant denominations may still claim that the Pope was the Antichrist but things have moved on since then. For example, one denomination says the harlot of Revelation, Babylon the Great, is all false religion. They also say that all Trinitarian churches are part of the Antichrist, thereby embracing most Protestant denominations as well as Catholicism.
Then we have a new interpretation of end-time events (1830’s onward) which proposes a pre-tribulation rapture of the elect followed by a literal 1,000 year reign of Christ Jesus physically seated in a new temple in Jerusalem and the restoration of Israel.
As a Christian of the Reformed Protestant persuasion, my study of Revelation indicates that the Antichrist has not yet been revealed. Many antichrists have arisen – take the Roman Emperors who persecuted the Christians of the first and second centuries. They were political enemies of the church (pre Constantine and Catholicism).
Since 70 C.E. persecution of God’s saints has continued unabated, at the hands of both religious and political enemies of Christianity, but the Antichrist has not yet appeared on the world scene. When he is revealed, the world will be amazed because he will perform miracles, usher in world peace and claim to be God, demanding the worship of every human. I don’t see any pope fitting that description, do you?
You ask, what suddenly changed that made all Protestants cease to claim the pope is the antichrist? I don’t know if that is the case, and if so, it certainly wasn’t sudden.
You ask why it is no longer an important doctrine. It’s not important because eternal salvation does not depend on interpretation of end-time prophecy. Christians who belong to Christ Jesus refuse to give worship to any creature other than Almighty God. They refuse to renounce their Lord and Saviour, even in the face of persecution and death. That’s been going on for 2,000 years and counting and will continue till the Lord Christ Jesus returns to destroy his enemies.