How central is the claim the pope is the antichrist to Protestant theology?

OP: So, it looks like until fairly recently (late 1800s, early 1900s) the pope being the antichrist was pretty important for most Protestants. Was that the case?

As noted throughout the answers, it was important and widespread.

Protestant Reformers, including John Wycliffe, Martin Luther, John Calvin, Thomas Cranmer, John Thomas, John Knox, Roger Williams, Cotton Mather, and John Wesley, as well as most Protestants of the 16th-18th centuries, felt that the Early Church had been led into the Great Apostasy by the Papacy and identified the Pope with the Antichrist.[59][60] Luther declared that not just a pope from time to time was Antichrist, but the Papacy was Antichrist because they were "the representatives of an institution opposed to Christ".[61] The Centuriators of Magdeburg, a group of Lutheran scholars in Magdeburg headed by Matthias Flacius, wrote the 12-volume Magdeburg Centuries to discredit the Catholic Church and lead other Christians to recognize the Pope as the Antichrist. So, rather than expecting a single Antichrist to rule the earth during a future Tribulation period, Martin Luther, John Calvin, and other Protestant Reformers saw the Antichrist as a present feature in the world of their time, fulfilled in the Papacy. Wiki-Antichrist

OP: If so, why was it so central?

It was central because in their view it was leading the church, the believer, away from the truth of the gospel.

For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: Not of works, lest any man should boast. Eph 2:8-9

Moreover, the well-known 95 Theses of Luther outline various other departures from the faith once delivered. For example, forbidding clergy to marry, paying for and requiring indulgences, doing penance, enforcing purgatory so-called, hoarding wealth, and representing the pope as head of the church.

OP: If it was so central, what suddenly changed that made all Protestants cease to claim the pope is the antichrist? Why is it no longer an important doctrine?

There are a couple of threads.

One is the view that, while the pope may have moved away, it is not necessarily to say Christ's sheep have moved away. The Protestant "love the person, not the sin" prevails.

Two is the Catholic easing of their claim to the sole way to salvation. This of course may be denied by many Catholics, especially post Vatican II.

Three is the weakening of the pope and the expansion of the grace, the gospel, message. While the Catholic Church remains the largest denomination, it is after all just that, a denomination of many.

To be clear, the doctrine per se, the identity of the man of sin, the anti-christ, the son of perdition is important still.

The claim that the papacy or the pope was the Anti-Christ was important to the early Reformers. Theologically, their focus was largely on issues such as the role of faith and scripture in salvation, as opposed to the authority of priests and the Roman Church. But in terms of ecclesiology per se, the corruption of the papacy was crucial. Both Luther and Calvin were specific in their accusation against the papacy as the Anti-Christ. Calvin stated:

Shall we recognise the Apostolic See where we see nothing but horrible apostacy? Shall he be the vicar of Christ who, by his furious efforts in persecuting the Gospel, plainly declares himself to be Antichrist? Shall he be the successor of Peter who goes about with fire and sword demolishing everything that Peter built? Shall he be the Head of the Church who, after dissevering the Church from Christ, her only true Head, tears and lacerates her members? Rome, indeed, was once the mother of all the churches, but since she began to be the seat of Antichrist she ceased to be what she was. (Institutes 4.7.25)

Luther has been quoted as saying:

We here are of the conviction that the papacy is the seat of the seed of the true and real antichrist. I owe the Pope no other obedience than that I owe to antichrist” (August 18th, 1520).

Luther even went so far as to say as much to the pope himself in his letter of 1520 addressed to Leo X

if a Pontiff rules while Christ is absent and does not dwell in his heart, what else is he but a vicar of Christ? And then what is that Church but a multitude without Christ? What indeed is such a vicar but Antichrist and an idol? How much more rightly did the Apostles speak, who call themselves the servants of a present Christ, not the vicars of an absent one.

Luther's followers would go even farther, aided by popular illustrations such as those of Cranach, depicting the popes as devils.

Presbyterians often shared this viewpoint. For example, the Dutch Reformer Wilhelmus à Brakel wrote:

Question: Who is the antichrist?

Answer: With all Protestants we reply: The pope of Rome. The papists deny this strongly.

Many later Protestants would echo these sentiments, accusing either the papacy or specific popes of being The Antichrist or representing "the spirit of anti-christ." (1 John 4:3). The Roman Church, meanwhile was often depicted as the Whore of Babylon. As the Reformation became firmly established, however, the Catholic Church posed less of a threat to Protestants. Military conflicts and mutual persecution between Catholic and Protestant forces became a thing of the past. Denunciation of Rome by Protestants became less important than missionary work and spreading the Gospel to the unconverted. In the 20th Century, Protestant-Catholic dialogues were initiated. Some denominations still actively denounce the papacy as the Anti-Christ, but this has increasingly become the exception rather than the norm.

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.

The idea that the Pope is the Antichrist is very much a fringe belief in Protestantism.

While historically many reformers accused the Pope of being the Antichrist, this is no longer the belief of the vast majority of Protestant denominations. Mainstream Lutherans, Anglicans, Baptists deny the authority of the Pope over Christians, and have strong disagreements over doctrine, but have abandoned the categorization of either Catholics or the Pope as Antichrist.

There are smaller breakaway subdenominations that do adhere to the belief of the Pope as Antichrist, and some of the use the names Lutheran or Baptist (and others).