How did the Gospel writers know that Mary was a virgin?

• According to sources (however with a reliability lower than the Gospels themselves) Mary has been alive for a sufficient time after ascension and she had contract to the Christian community .
• Most Apostles came from a place close to Nazareth and stayed with Jesus so they may have known it early.
• The Gospel writers never mentioned the source of their knowledge so that there's no evidence to give an exact answer to the question.

In brief: They can have had the information but we don't know from whom.

How did Gospel writers know that Mary was a virgin?

Either they talked to Mary, or God communicated it to them directly. But which one is it?

The virgin birth of Jesus is the Christian doctrine that Jesus was conceived and born by his mother Mary through the power of the Holy Spirit and without sexual intercourse. It is mentioned only in Matthew 1:18-25 and Luke 1:26-38, and the modern scholarly consensus is that the narrative rests on very slender historical foundations.

The ancient world had no understanding that male s*m*n and female ovum were both needed to form a fetus; this cultural milieu was conducive to miraculous birth stories, and tales of virgin birth and the impregnation of mortal women by deities were well known in the 1st-century Greco-Roman world and Second Temple Jewish works. Matthew and Luke use the virgin birth (or more accurately the divine conception that precedes it) to mark the moment when Jesus becomes the Son of God, in distinction to Mark, for whom the Sonship dates from Jesus's baptism,[Mark 1:9-13] and Paul and the pre-Pauline Christians for whom Jesus becomes the Son only at the Resurrection or even the Second Coming.

The Eastern Orthodox Churches accept the doctrine as authoritative by reason of its inclusion in the Nicene Creed, and the Catholic Church likewise holds it authoritative for faith through the Apostles' Creed as well as the Nicene. Christians, including Protestants, traditionally regard it as an explanation of the mixture of the human and divine natures of Jesus. Nevertheless, there are many contemporary churches in which it is considered orthodox to accept the virgin birth but not heretical to deny it.

The Gospels of Matthew and Luke agree that Mary's husband was named Joseph, that he was of the Davidic line, and that he played no role in Jesus's divine conception, but beyond this they are very different. Matthew underlines the virginity of Mary by references to the Book of Isaiah (using the Greek translation in the Septuagint, rather than the mostly Hebrew Masoretic Text) and by his narrative statement that Joseph had no sexual relations with her until after the birth (a choice of words which leaves open the possibility that they did have relations after that).

Historicity and sources of the narratives

In the entire Christian corpus the virgin birth is found only in the Gospel of Matthew and the Gospel of Luke, and the modern scholarly consensus is that the narrative rests on very slender historical foundations. Matthew uses Isaiah 7:14 to support his narrative, but scholars agree that the Hebrew word used in Isaiah, almah, signifies a girl of childbearing age without reference to virginity, and was aimed at Isaiah's own immediate circumstances. In the John, slightly later than Matthew and Luke, Jesus has both father and mother, ("We have found him about whom Moses in the law and also the prophets wrote, Jesus, son of Joseph from Nazareth") and his conception does not entail divine intervention.

Mary, the Mother of Jesus, lived for many years after the death and resurrection of her divine Son. Wikipedia suggests that Mary died around 41 AD, while others place her death somewhat later. One tradition says that Mary died in 43 AD and another in 48 AD, but we have no way of confirming either date. (When did Mary die?) I have even seem the year of her death as being in 54 AD.

What is my point in all this. Mary had plenty of time to explain the circumstances of the birth of Jesus to the Evangelists in person.

Sure God could of infused this knowledge into the minds of the Evangelists. However tradition goes that the circumstances of Jesus’s birth came from the lips of his mother herself.

St. Luke got the greatest interview of all time

He begins his Gospel telling us he's done his homework and talked to eyewitnesses.

There’s a school of thought in biblical studies that each of the four Gospels was written for a particular audience: Matthew for the Jews, Mark for Roman Christians, Luke for Gentiles (especially Greeks), and John for a later Christian audience already familiar with the story of Jesus.

There are indicators throughout each to support this theory. Matthew’s Gospel is filled with allusions to the Old Testament prophets and seems to be trying to persuade a Jewish audience that Jesus is the Messiah they have been waiting for.

Mark’s Gospel begins and ends with an affirmation that Jesus is “the Son of God,” in contrast to the Roman emperor, who styled himself divi filius.

Luke writes in the mode of the Greek historians like Herodotus and Thucydides.

And John’s is the most theologically complex of all the Gospels.

Luke begins his Gospel with a statement of intent: He tells Theophilus that he has endeavored to “compile a narrative of the things which have been accomplished among us, just as they were delivered to us by those who from the beginning were eyewitnesses and ministers of the word” (Luke 1:1-2). Compare this with the beginning of Herodotus’ The Histories: “These are the researches of Herodotus of Halicarnassus, which he publishes, in order to preserve from oblivion the remembrance of former deeds of men.” Both men are establishing their credibility as historians by claiming essentially that they’ve done their homework, by “researches” and talking with “eyewitnesses.”

It is especially important for us as Christians that Luke states he has composed his narrative by talking with those who were involved in the life and ministry of Jesus “from the beginning.” Luke himself was not an apostle; as far as we know, he never knew Jesus. Matthew and John, of course, were two of the Twelve, but Mark and Luke were traveling companions of St. Paul, as Luke tells us himself in the Acts of the Apostles. (Mark also traveled with Peter.) Thus Luke would have gained his knowledge of Jesus from the apostles, from sitting at their feet and hearing their preaching.

Each of the Gospels has unique material, stories, and parables that do not appear anywhere else. Yet some of the content of Luke is truly remarkable when we think about it. It’s one thing for Matthew or John to bring forward different stories from their time with Jesus, or for Mark to recount some memory of Peter. But some of the details in Luke likely would not have come from the apostles, since Luke records events that predate their arrival on the scene by quite a bit. I’m referring, of course, to the events surrounding the birth of Jesus.

True, Matthew’s Gospel does give us a brief account of the circumstances of the birth of Jesus, and of the flight into Egypt (which is unique to his Gospel), but Luke gives us much greater detail. Luke tells us of the visits of the Archangel Gabriel both to Zechariah and to the Blessed Virgin Mary. Luke gives us a precise recounting of the birth of Jesus: the Roman census, the journey to Bethlehem, the visit of the shepherds—even the intimate moment of the child being wrapped in swaddling clothes and laid in a manger. He also tells of Jesus’ circumcision as an infant and the finding in the temple at the age of 12. Where did Luke get all of this extraordinary detail? The key is in Luke 2:51— ”His mother treasured all these things in her heart.”