The Orthodox Church has always proclaimed the Ever-Virginity of Mary, the Mother of our God (Theotokos). The verse referring to Mary's "Firstborn" has been misinterpreted by a countless number of the heterodox.
"...and he knew her not up to [Greek = eos] her having brought forth her firstborn son..." (Matthew 1:25; Luke 2:7).
This verse seems to be often translated as "he knew her not until after..." This is not, however, what is meant. The Greek original, "eos", indicates the true meaning, of "he had no sexual relations with her prior to her giving birth." The Evangelist makes this statement in order to assure us that Joseph had no part in the conception of Jesus. The term eos ou does not require the understanding that he had relations with her after Christ was born. It merely indicates that, as regards the birth of Jesus, Joseph had not had relations with Mary prior to the birth, thus, he was not the father of Jesus. This is merely a usual turn of phrase, the use of a standard and familiar form of expression. This same term and meaning is used elsewhere in the Bible as a standard expression, and it clearly does not indicate what the heterodox (non-Orthodox) claim it does. At 2 Samuel 6:23, for instance, we read, "And Milchal, the daughter of Saul, had no child until [eos] her death. Did she, then, have children after her death? Of course not!, and neither did Joseph "know" Mary after the birth of Jesus. At Genesis 8:7, we read that Noah "sent forth a raven; and it went forth and did not return till [eos] after the water had gone from off the face of the earth." We know from Scripture that in fact, the raven never returned to the ark. It says that it did not return "until after," but in fact, it never returned at all. The Scripture says that "Joseph knew her not till after...", but in fact, he never "knew" her at all. In another example, the Bible says, 'The Lord said to my Lord, 'Sit at my right hand until [eos] I make Thine enemies Thy footstool" (Mark 12:36). Does this mean that Christ will cease to sit at the right hand of the glory of the Father once His enemies have been overcome? Of course not ! Hence, the Bible does not say that "Joseph knew her not until after she brought forth her first born, but then he did." The Bible says, "He did not know her before (up until) she had brought forth her firstborn," meaning simply and clearly, "Joseph was not the father. He had not come together with her before her pregnancy, thus he was not involved in the conception of Jesus."
The Sacred Tradition concerning these matters is certainly derived from Mary's own testimony and, like all things in the Church, it is guided and guarded by the indwelling Holy Spirit. Mary was, in fulfilment of the Mosaic Law, betrothed to Joseph, an older man who was in fact her own uncle (as the Holy Spirit guided Sacred Tradition tells us). Thus their "marriage" was a marriage which, according to the Mosaic Law did not allow for sexual intercourse between them, because she was the bearer of the inheritance, her Firstborn, the Messiah. The term Firstborn means simply that. It does not imply any further births, but simply describes the first. In the Mosaic law the first born of any female (human or even animal) has religious significance, and this is why Jesus as Firstborn is emphasised.
Finally, you might ask "how is it physically possible for Mary to remain a virgin after the birth of Christ ?" The simple answer, as given in the Scriptures is "With men this is impossible, but with God ALL things are possible." (Matt. 19:26)
Who were the "brethren of the Lord" (Matthew 12:46-47), and if He had brothers, why do we call the Theotokos "Ever-Virgin" ?
The "brethren" of Jesus are mentioned several times in the New Testament. Four are mentioned by name. To explain who they were is not difficult, because the Scripture itself names four of them and identifies their parentage. Matthew (13:55) and Mark (6:3) list, as brethren of Jesus, James, Joses, Simon and Jude.
We know for certain that James and Joses were not sons of Mary or Joseph, for the Scripture identifies them, as children of a different Mary, who was the wife of Alphaeus-Cleopas (Matthew 27:56 ; Mark 15:40). James is also referred to as the "son of Alphaeus", in the listing of the Apostles (Matthew 10:3 ; Mark 3:18 ; Luke 6:15 ; Acts 1:13). The relationship between these "brethren" (including "sisters") must be seen in the context of Hebrew-Aramaic tradition, according to which even cousins were called brothers and sisters. This is the case also in Greek and Slavic languages and cultures to this day, so we do not have to speculate about it. This is a fact we know very well from our own families and lives. We have a perfect example of this in the Old Testament Scripture. The word used to describe the relationship between Lot and Abraham at Genesis 14:16 is "adelphi" in the original Greek, which can only be translated as "brother" in English. Nevertheless, we know that Lot was Abraham's nephew. The Greek word "adelphos" and "adelphi" are only attempts to translate an unknown Aramaic word - and no one has any idea what the actual word was which is rendered in Greek and English as "brothers" or "brethren".
There could have been no "first blood" brothers of Christ, otherwise He would not have given the care of His mother to St. John the Theologian (John 19:26) at the foot of the Cross. Indeed, Christ would have done His 'brothers' great disrespect and harm if He had done this ! The Old Testament prophecies explain the virginal marriage and ever-virginity of Christ's mother, and we also have the testimony of the Holy Spirit speaking through the Church that Mary is "Ever-Virgin".
Further evidence from the Holy Scriptures that in the Hebrew tradition "brothers" and "sisters" are not necessarily siblings. Our Orthodox Tradition teaches us that the Holy Virgin Mary was the only child of Saints Joakhim and Anna, but at John 19:25 we read, "Standing near the Cross of Jesus was His mother, and His mother's sister, Mary of Klopas, and Mary magdala." If our Church history is correct, how could Mary have had a sister? The first clue to our answer is that both women are named Mary. ! No family has two daughters and gives them both the same name! Therefore it is evident that the relationship between the two women has to be something different than our modern English concept of "sister". The second clue to our answer is that the Bible clearly identifies this Mary of Klopas (Cleopas in KJV), as the mother of Jesus' "brothers". The name Klopas or Cleopas is the same as Alphaeus in the Aramaic language which Jesus spoke. Therefore the so-called brothers of Jesus mentioned at Mark 6:3 are elsewhere clearly identified as the sons of Alphaeus and his wife Mary of Klopas - the "sister" of the Virgin Mary.
Thus the Scriptures show that the "brothers" of Christ are not His brothers, but some relation. There is no scriptural evidence to support the notion that the Virgin Mary bore any other children apart from Jesus Christ our God.
Seeing Him born as an infant in Bethlehem. Let all creation glorify Him!
All Glory be to Jesus Christ our God.
The Ever-Virginity of Mary, the Title Theotokos, and the veneration shown to Her
published by Synaxis Press and translated into French here.