The perpetual virginity of Mary is nowhere found in the Bible.
Can you prove anyone believed this prior to the third or fourth century?
And, hey, saying protestant beliefs did not exist prior to the 2nd century is insane.
If Protestant beliefs existed that early, you should have no problem finding those people then. I look forward to what you find.
Here are just some early testaments to Mary’s perpetual virginity.
ORIGEN OF ALEXANDRIA
But some say, basing it on a tradition in the Gospel according to Peter, as it is called, or “The Book [Protoevangelium] of James,” that the brothers of Jesus were sons of Joseph by a former wife, whom he married before Mary. Now those who say so wish to preserve the honor of Mary in virginity to the end, so that her body, which was appointed to minister to the Word, which said, “The Holy Spirit shall come upon you, and the power of the Most High shall overshadow you” [Lk 1:35], might not know intercourse with a man after the Holy Spirit came into her and the power from on high overshadowed her. And I think it reasonable that Jesus was the first fruit among men of the purity that consists in chastity, and Mary among women; for it is not pious to ascribe to any other than her the first fruit of virginity [Commentary on Matthew 10:17 (c. A.D. 249)].
ST. ATHANASIUS OF ALEXANDRIA
Therefore let those who deny that the Son is from the Father by nature and proper to his essence deny also that he took true human flesh of Mary Ever-Virgin [Four Discourses Against the Arians 2:70 (c. A.D. 360)].
ST. JEROME
Now that I have cleared the rocks and shoals I must spread sail and make all speed to reach his epilogue. Feeling himself to be a smatterer, he there produces Tertullian as a witness and quotes the words of Victorinus, bishop of Petavium. Of Tertullian I say no more than that he did not belong to the Church. But as regards Victorinus, I assert what has already been proved from the Gospel—that he spoke of the brothers of the Lord not as being sons of Mary, but brethren in the sense I have explained, that is to say, in point of kinship, not by nature. We are, however, spending our strength on trifles, and, leaving the fountain of truth, are following tiny streams of opinion. Might I not array against you the whole series of ancient writers? Ignatius, Polycarp, Irenaeus, St. Justin Martyr, and many other apostolic and eloquent men, who against Ebion, Theodotus of Byzantium, and Valentinus, held these same views, and wrote volumes full of wisdom. If you read what they wrote, you would be a wiser man. But I think it better to reply briefly to each point than to linger any longer and extend my book to an undue length [Perpetual Virginity of Blessed Mary 19 (A.D. 383)]. We believe that God was born of the Virgin, because we read it. That Mary was married after she brought forth, we do not believe, because we do not read it. Nor do we say this to condemn marriage, for virginity itself is the fruit of marriage; but because when we are dealing with saints we must not judge rashly. If we adopt possibility as the standard of judgment, we might maintain that Joseph had several wives because Abraham had, and so had Jacob, and that the Lord’s brothers were the issue of those wives, an invention that some hold with a rashness that springs from audacity, not piety. You say that Mary did not continue a virgin: I claim still more that Joseph himself, on account of Mary, was a virgin, so that from a virgin wedlock a virgin son was born [ibid., 21].
POPE ST. SIRICIUS I
Surely, we cannot deny that regarding the sons of Mary the statement is justly censured, and your holiness rightly abhors it, that from the same virginal womb from which Christ was born, another offspring was brought forth. For neither would the Lord Jesus have chosen to be born of a Virgin if he had judged she would be so incontinent, that with the seed of human copulation she would pollute the generative chamber of the Lord’s body, the palace of the eternal king [Letter to Bishop Anysius (A.D. 392)].
ST. AMBROSE OF MILAN
Imitate [Mary], holy mothers, who in her only dearly beloved Son set forth so great an example of maternal virtue; for neither have you sweeter children, nor did the Virgin seek the consolation of being able to bear another son [Letters 63:111 (A.D. 396)].
ST. AUGUSTINE OF HIPPO
Thus Christ by being born of a Virgin who, before she knew who was to be born of her, had determined to continue a Virgin, chose to approve, rather than to command, holy virginity. And thus, even in the female herself, in whom he took the form of a servant, he willed that virginity should be free [Holy Virginity 4:4 (A.D. 401)].
LEPORIUS
We confess, therefore, that our Lord and God, Jesus Christ, the only Son of God, born of the Father before the ages, and in times most recent, made man of the Holy Spirit and the Ever-Virgin Mary [Document of Amendment 3 (A.D. 426)].
POPE ST. LEO I
The origin is different but the nature alike: not by intercourse with man but by the power of God was it brought about: for a Virgin conceived, a Virgin bore, and a Virgin she remained [Sermons 22:2 (A.D. 450)].
COUNCIL OF CONSTANTINOPLE II
If anyone will not confess that the Word of God . . . came down from the heavens and was made flesh of holy and glorious Mary, Mother of God and Ever Virgin, and was born from her, let him be anathema [Capitula of the Council 2 (A.D. 553)].