One of the major, early catalysts in the Protestant Reformation was a book by John Huss, a Bohemian Christian who preceded Martin Luther by a full century. The book was De Ecclesia (The Church), and one of Huss’s most profound points was proclaimed in the title of his fourth chapter: “Christ the Only Head of the Church.”
Huss wrote, “Neither is the pope the head nor are the cardinals the whole body of the holy, universal, catholic [i.e., true] church. For Christ alone is the head of that church.”  John Huss, The Church, translated by David Schaff (New York, NY: Charles Scribner’s Sons, 1915), 66. Pointing out that most church leaders in his era actually despised the lordship of Christ, Huss said, “To such a low pitch is the clergy come that they hate those who preach often and call Jesus Christ Lord.”  Huss, The Church, 291.
Huss’s candor cost him his life. He was declared a heretic and burned at the stake in 1415.
More than a hundred years later, and already at odds with the papal establishment, Martin Luther read De Ecclesia. After finishing the book, he wrote to a friend, “I have hitherto taught and held all the opinion of John Huss unawares; so did John Staupitz; in short, we are all Hussites without knowing it.”  Martin Luther, The Life and Letters of Martin Luther, translated by Preserved Smith (Cambridge, UK: Houghton Mifflin Company, 1914), 72.
As the head of the Roman Catholic Church, the pope is often called the “Holy Father” and the “Vicar of Christ”—names and roles that apply only to God. He claims the ability to speak ex cathedra, exercising godlike infallibility to add to and augment Scripture (Revelation 22:18). He wields unbiblical, unholy authority over his followers, usurping the headship of Christ and perverting the work of the Holy Spirit.
The Reformers understood that and declared it with unashamed boldness. As Martin Luther wrote to a friend,
We here are of the conviction that the papacy is the seat of the true and real Antichrist, against whose deceit and vileness all is permitted for the salvation of souls. Personally I declare that I owe the Pope no other obedience than that to Antichrist.  Martin Luther, cited in Ronald Charles Thompson, Champions of Christianity: In Search of Truth, Volume 1 in the series Genuine Christianity (Brushton, NY: Teach Services Inc., 1996), 82.
In his Institutes of the Christian Religion, John Calvin said:
Some persons think us too severe and censorious, when we call the Roman pontiff Antichrist. But those who are of this opinion do not consider that they bring the same charge of presumption against Paul himself, after whom we speak, and whose language we adopt. And lest anyone should object, that we improperly pervert to the Roman pontiff those words of Paul, which belong to a different subject, I shall briefly show that they are not capable of any other interpretation than that which implies them to the papacy  John Calvin, Institutes of the Christian Religion, translated by John Allen (Philadelphia, PA: Presbyterian Board of Publication, 1843), 2:335.
The words of Paul that Calvin referred to were from 2 Thessalonians, where the apostle described the coming Antichrist “who opposes and exalts himself above every so-called god or object of worship, so that he takes his seat in the temple of God, displaying himself as being God” (2 Thessalonians 2:4).
That same understanding was later reflected in the Westminster Confession of Faith, which says,
There is no other head of the church but the Lord Jesus Christ. Nor can the Pope of Rome, in any sense, be head thereof; but is that Antichrist, that man of sin and son of perdition, that exalts himself in the church against Christ and all that is called God. Westminster Confession of Faith, Chapter 25, Paragraph 6.
That doesn’t mean that the pope is the final Antichrist. There have been and will continue to be, as 1 John 2:18 says, many false teachers who embody the spirit of Antichrist. As the American Puritan Cotton Mather wrote in The Fall of Babylon,
The oracles of God, foretold, Rising of an ANTICHRIST [i.e., one or more antichrists who embody the spirit of the final Antichrist] in the Christian church; And in the POPE of Rome, all the Characters of that Antichrist, are so marvellously answered that if any who read the Scriptures do not see it, there is a Marvellous Blindness upon them. Cotton Mather, cited in Leroy Edwin Froom, The Prophetic Faith of Our Fathers: Volume III (Washington, DC: Review and Herald, 1946), 154.
In a sermon titled “Pray for Jesus,” Charles Haddon Spurgeon exhorted his congregation that
it is the duty of every Christian to pray against Antichrist, and as to what Antichrist is. No sane man ought to raise a question. If it be not the popery in the church of Rome and in the church of England, there is nothing in the world that can be called by that name.  Charles Spurgeon, The Complete Works of Charles Spurgeon: Volume 12, originally published 1866 (Harrington, DE: Delmarva Publications, 2013), #717.
He went on to say:
Popery anywhere, whether it be Anglican or Romish, is contrary to Christ’s gospel! And it is the Antichrist, and we ought to pray against it! It should be the daily prayer of every believer that Antichrist might be hurled like a millstone into the flood and sink to rise no more. If we can pray against error for Christ because it wounds Christ, because it robs Christ of His glory, because it puts sacramental efficacy in the place of His atonement and lifts a piece of bread into the place of the Savior, and a few drops of water into the place of the Holy Spirit, and puts a mere fallible man like ourselves up as the Vicar of Christ on earth—if we pray against it because it is against Him—we shall love the persons though we hate their errors! We shall love their souls though we loathe and detest their dogmas, and so the breath of our prayers will be sweetened because we turn our faces toward Christ when we pray.  Spurgeon, The Complete Works of Charles Spurgeon: Volume 12, #717.
In another sermon, titled “Christ Glorified,” Spurgeon said:
Christ did not redeem His church with His blood so the pope could come in and steal away the glory. He never came from heaven to earth and poured out His very heart that He might purchase His people so that a poor sinner, a mere man, should be set upon high to be admired by all the nations and to call himself God’s representative on earth! Christ has always been the head of His church.  Charles Spurgeon, The Complete Works of Charles Spurgeon: Volume 60, originally published 1914 (Harrington, DE: Delmarva Publications, 2013), #3436.
In 1 Timothy 2:5, Paul said, “For there is one God, and one mediator also between God and men, the man Christ Jesus.” The pope has assumed for himself a position of authority that is not recognized by God.