Was Jesus born of a virgin?

Ignatius of Antioch d. c. 110 ADLetter to the Ephesians 19.1

The virginity of Mary and her giving birth were hidden from the ruler of this age, as was the death of the Lord: three mysteries to be shouted aloud, which were done in God's silence.

Ignatius treats the virgin birth as one of the three central mysteries, alongside the cross.

Justin Martyr c. 100–c. 165 ADFirst Apology 33.1–4

Hear how it was foretold through Isaiah, in so many words, that he would be born of a virgin. It was said: "Behold, the virgin shall conceive in the womb and bear a son, and they shall say of his name, God with us" (Isaiah 7:14). For God foretold through the prophetic Spirit that things thought unbelievable and impossible among men would happen, so that when they happened they would not be disbelieved. ... "Behold, the virgin shall conceive" means that the virgin conceived without intercourse.

Justin explains the virgin birth to pagan readers and distinguishes it from myths of gods lying with women.

Trypho (Jewish critic), in Justin's DialogueDialogue with Trypho 67.1

And Trypho answered: The Scripture does not have "Behold, the virgin shall conceive and bear a son" but "Behold, the young woman shall conceive and bear a son," and so on as you said. The whole prophecy was spoken of Hezekiah, and it is shown to have come true in him.

The debate over Isaiah 7:14 (Hebrew almah, Greek parthenos) already ran in the second century. Justin answers in the chapters that follow.

Celsus (pagan critic), quoted by OrigenTrue Word, quoted in Origen, Against Celsus 1.32

Let us go back to the words put in the mouth of Celsus's Jew, where it is written that the mother of Jesus was driven out by the carpenter who was engaged to her, convicted of adultery, and bore a child by a certain soldier named Panthera.

The earliest pagan attack on the virgin birth. It shows the Christian claim was well known by about 178. Origen answers it.

Irenaeus of Lyons (quoted by Eusebius) c. 130–c. 202 ADAgainst Heresies, quoted in Eusebius, Church History 3.21.1

God became man, and the Lord himself saved us, giving the sign of the Virgin. But it is not as some now dare to translate the Scripture, Behold, a young woman shall conceive and bear a son, as Theodotion the Ephesian translated it, and Aquila of Pontus, both Jewish proselytes, whom the Ebionites followed in saying that he was begotten of Joseph.

Irenaeus defends the old Greek version of Isaiah 7:14, which reads virgin (parthenos), against the newer Jewish versions of Theodotion and Aquila, which read young woman (neanis). Eusebius quotes the passage in full; the Greek of Against Heresies 3.21 survives in his quotation.