It was always central to God’s purpose to provide his Son as the Saviour.
Paul makes clear that this Son was a human being born of a woman:
4 But when the fullness of the time came, God sent out his Son, born to a woman, born under the law,
There had been a promise in Eden of a special descendant of the woman (Eve):
15 I will put hostility between you and the woman,
and between your offspring and her offspring.
He will bruise your head,
and you will bruise his heel.”
The prophet Isaiah gave more detail of how this would happen:
14 Therefore the Lord himself will give you a sign. Behold, the virgin will conceive, and bear a son, and shall call his name Immanuel.
The gospel records are very explicit that Jesus was born of a virgin, whose name was Mary:
18 Now the birth of Jesus Christ was like this: After his mother, Mary, was engaged to Joseph, before they came together, she was found pregnant by the Holy Spirit. 19 Joseph, her husband, being a righteous man, and not willing to make her a public example, intended to put her away secretly. 20 But when he thought about these things, behold, an angel of the Lord appeared to him in a dream, saying, “Joseph, son of David, don’t be afraid to take to yourself Mary, your wife, for that which is conceived in her is of the Holy Spirit. 21 She shall give birth to a son. You shall name him Jesus, for it is he who shall save his people from their sins.”
That Joseph was not the father is also explicitly stated:
24 Joseph arose from his sleep, and did as the angel of the Lord commanded him, and took his wife to himself; 25 and didn’t know her sexually until she had given birth to her firstborn son. He named him Jesus.
The angel Gabriel explained to Mary how the conception and birth would come about:
26 Now in the sixth month, the angel Gabriel was sent from God to a city of Galilee named Nazareth, 27 to a virgin pledged to be married to a man whose name was Joseph, of David’s house. The virgin’s name was Mary.
34 Mary said to the angel, “How can this be, seeing I am a virgin?”
35 The angel answered her, “The Holy Spirit will come on you, and the power of the Most High will overshadow you. Therefore also the holy one who is born from you will be called the Son of God.