The Perpetual Virginity of Mary
The perpetual virginity of Mary means that the Catholic Church believes that Mary was a virgin before, during and after the birth of Christ. In the official catechism of the Catholic church from the Vatican, which can be found here , In part 1, section 2, chapter 2, article 3, paragraph 2, number 499 they state this belief.
499 The deepening of faith in the virginal motherhood led the Church to confess Mary's real and perpetual virginity even in the act of giving birth to the Son of God made man. In fact, Christ's birth "did not diminish his mother's virginal integrity but sanctified it." and so the liturgy of the Church celebrates Mary as Aeiparthenos, the "Ever-virgin".
They also claim in number 501:
501 Jesus is Mary's only son.
The Bible clearly teaches that Mary had children with Joseph after Jesus was born.
After this he went down to Capernaum, with his mother and his brothers and his disciples, and they stayed there for a few days. (John 2:12)
4While he was still speaking to the people, behold, his mother and his brothers stood outside, asking to speak to him. 48But he replied to the man who told him, "Who is my mother, and who are my brothers?" 49And stretching out his hand toward his disciples, he said, "Here are my mother and my brothers! 50For whoever does the will of my Father in heaven is my brother and sister and mother." (Mathew 12:46-50)
After this Jesus went about in Galilee. He would not go about in Judea, because the Jews were seeking to kill him. 2Not the Jews’ Feast of Booths was at hand. 3 So his brothers said to him, "Leave here and go to Judea, that your disciples also may see the works you are doing. (John 7:1-3)
All these with one accord were devoting themselves to prayer, together with the women and Mary the mother of Jesus, and) his brothers. (Acts 1:14)
18Then after three years I went up to Jerusalem to visit Cephas and remained with him fifteen days. 19But I saw none of the other apostles except James the Lord’s brother. (Galatians 1:18-19)
Is not this the carpenter’s son? Is not his mother called Mary? And are not his brothers James and Joseph and Simon and Judas? 56And are not all his sisters with us? (Matthew 13:55-56)
It is quite odd with all the references to Mary having other children, and now the Bible is naming them, that the Catholic Church holds to the doctrine of Mary as a perpetual virgin.
To justify these verses of Scripture that clearly state Mary had other children after the birth of Jesus; the Catholic church invents new doctrine in number 500 of their catechism:
500 Against this doctrine the objection is sometimes raised that the Bible mentions brothers and sisters of Jesus. The Church has always understood these passages as not referring to other children of the Virgin Mary. In fact James and Joseph, "brothers of Jesus", are the sons of another Mary, a disciple of Christ, whom St. Matthew significantly calls "the other Mary". They are close relations of Jesus, according to an Old Testament expression.
Notice how they can’t defend their doctrine using Scripture so they use the tradition of the Catholic church, which is exalted higher than Scripture. They ignore what the Bible says and stand on how the Catholic church has always understood these passages. Sometimes the Catholic church will say that “brothers” and “sisters” were general terms that really could refer to any kind of kinsman, including cousins. This is true in the Hebrew language. However, the New Testament is written in Greek, which is an extremely precise language. It makes a clear distinction between the words used to describe family relationships.