Marcion of Pontus, at the end of the first century AD, considering that Jesus proposed to improve on the Old Testament, and considering the nature of some of the content found in the Old Testament, proposed a radical break between the New and Old Testaments. His writings are lost, but descriptions of his theory remain. St. Irenaeus says in Against Heresies:

Cerdo was one who took his system from the followers of Simon, and came to live at Rome in the time of Hyginus, who held the ninth place in the episcopal succession from the apostles downwards. He taught that the God proclaimed by the law and the prophets was not the Father of our Lord Jesus Christ. For the former was known, but the latter unknown; while the one also was righteous, but the other benevolent.

Marcion of Pontus succeeded him, and developed his doctrine. In so doing, he advanced the most daring blasphemy against Him who is proclaimed as God by the law and the prophets, declaring Him to be the author of evils, to take delight in war, to be infirm of purpose, and even to be contrary to Himself. But Jesus being derived from that father who is above the God that made the world, and coming into Judæa in the times of Pontius Pilate the governor, who was the procurator of Tiberius Cæsar, was manifested in the form of a man to those who were in Judæa, abolishing the prophets and the law, and all the works of that God who made the world, whom also he calls Cosmocrator. Besides this, he mutilates the Gospel which is according to Luke, removing all that is written respecting the generation of the Lord, and setting aside a great deal of the teaching of the Lord, in which the Lord is recorded as most dearly confessing that the Maker of this universe is His Father. He likewise persuaded his disciples that he himself was more worthy of credit than are those apostles who have handed down the Gospel to us, furnishing them not with the Gospel, but merely a fragment of it. In like manner, too, he dismembered the Epistles of Paul, removing all that is said by the apostle respecting that God who made the world, to the effect that He is the Father of our Lord Jesus Christ, and also those passages from the prophetical writings which the apostle quotes, in order to teach us that they announced beforehand the coming of the Lord.

Similarly, Tertullian says in his work Against Marcion,

The heretic of Pontus introduces two Gods, like the twin Symplegades of his own shipwreck: One whom it was impossible to deny, i.e. our Creator; and one whom he will never be able to prove, i.e. his own god. The unhappy man gained the first idea of his conceit from the simple passage of our Lord’s saying, which has reference to human beings and not divine ones, wherein He disposes of those examples of a good tree and a corrupt one; how that the good tree brings not forth corrupt fruit, neither the corrupt tree good fruit. Which means, that an honest mind and good faith cannot produce evil deeds, any more than an evil disposition can produce good deeds. Now (like many other persons now-a-days, especially those who have an heretical proclivity), while morbidly brooding over the question of the origin of evil, his perception became blunted by the very irregularity of his researches; and when he found the Creator declaring, I am He that creates evil, Isaiah 45:7 inasmuch as he had already concluded from other arguments, which are satisfactory to every perverted mind, that God is the author of evil, so he now applied to the Creator the figure of the corrupt tree bringing forth evil fruit, that is, moral evil, and then presumed that there ought to be another god, after the analogy of the good tree producing its good fruit. Accordingly, finding in Christ a different disposition, as it were— one of a simple and pure benevolence — differing from the Creator, he readily argued that in his Christ had been revealed a new and strange divinity; and then with a little leaven he leavened the whole lump of the faith, flavouring it with the acidity of his own heresy.

He had, moreover, in one Cerdon an abettor of this blasphemy—a circumstance which made them the more readily think that they saw most clearly their two gods, blind though they were; for, in truth, they had not seen the one God with soundness of faith. To men of diseased vision even one lamp looks like many. One of his gods, therefore, whom he was obliged to acknowledge, he destroyed by defaming his attributes in the matter of evil; the other, whom he laboured so hard to devise, he constructed, laying his foundation in the principle of good. In what articles he arranged these natures, we show by our own refutations of them.

Apart from theological problems with asserting the existence of multiple gods, a major problem with this theory is that it is contrary to Jesus’s own words about himself, as when he says, “Do not think that I have come to abolish the law or the prophets; I have come not to abolish but to fulfill.”

Consequently, as Irenaeus says in the quoted passage, it was necessary for Marcion to eliminate a great deal from his canon of Scripture. Of the Gospels, he accepted only that of Luke, and had to do a great deal of editing to make it conform to his opinion. He did the same thing with the epistles of St. Paul.

Earlier we quoted Newman on Protestantism:

Meanwhile, before setting about this work, I will address one remark to Chillingworth and his friends:—Let them consider, that if they can criticize history, the facts of history certainly can retort upon them. It might, I grant, be clearer on this great subject than it is. This is no great concession. History is not a creed or a catechism, it gives lessons rather than rules; still no one can mistake its general teaching in this matter, whether he accept it or stumble at it. Bold outlines and broad masses of colour rise out of the records of the past. They may be dim, they may be incomplete; but they are definite. And this one thing at least is certain; whatever history teaches, whatever it omits, whatever it exaggerates or extenuates, whatever it says and unsays, at least the Christianity of history is not Protestantism. If ever there were a safe truth, it is this.

And Protestantism has ever felt it so. I do not mean that every writer on the Protestant side has felt it; for it was the fashion at first, at least as a rhetorical argument against Rome, to appeal to past ages, or to some of them; but Protestantism, as a whole, feels it, and has felt it. This is shown in the determination already referred to of dispensing with historical Christianity altogether, and of forming a Christianity from the Bible alone: men never would have put it aside, unless they had despaired of it. It is shown by the long neglect of ecclesiastical history in England, which prevails even in the English Church. Our popular religion scarcely recognizes the fact of the twelve long ages which lie between the Councils of Nicæa and Trent, except as affording one or two passages to illustrate its wild interpretations of certain prophesies of St. Paul and St. John. It is melancholy to say it, but the chief, perhaps the only English writer who has any claim to be considered an ecclesiastical historian, is the unbeliever Gibbon. To be deep in history is to cease to be a Protestant.

According to Newman, if you wish to take seriously the religion proposed by Christ and the Apostles, you have to take seriously the Christianity of the first, second, and third centuries, which leads right to the requirement that you take seriously the Christianity of the fourth, fifth, and sixth centuries, and so on. Taking this process seriously without inserting some radical break means accepting medieval Christianity, the Council of Trent, and Catholicism. To the degree that Newman is right, in order to be a Protestant one must insert some radical break into history. In this sense, Marcion could be said to be one of the original Protestants. Just as the reformers said, “Look at how corrupt the Church of Rome is, we need a real break from that,” Marcion said, “Look at how corrupt the Old Testament is, we need a real break from that.”

There is a similar problem with both attempted ruptures. Christ fully accepted the Old Testament, and in fact says that “the scripture cannot be annulled,” so rejecting the Old Testament means rejecting the religion of Christ. Similarly, the Protestant rejection of Catholicism implies rejecting historical Christianity, which likewise implies rejecting the religion of Christ.