From a human point of view, it might have been reasonable for Mary to lie. It would not have been right; it would not have been moral; but it would not have been surprising. She was promised in marriage to Joseph. They called it betrothal which was legally the same as being married. To break a betrothal was divorce. Infidelity was a cause to break the betrothal, and Mary was pregnant. She knew the baby was not Joseph's, but how could she explain it? Who would believe her? So, it seems plausible that Mary could have made up the story about the visit from an angel and a pregnancy resulting from a visit by the Holy Spirit. It would have been a far fetched story, but possible.
But then there is Joseph's account. He had no reason to lie. He was well within his rights to divorce Mary. He knew the baby was not his. He could have made a scene. He could probably have demanded Mary be stoned. But, being righteous, he chose instead to secretly “put her away” … until he was visited by the angel in a dream. The angel said to him, “Joseph, son of David, do not be afraid to take Mary as your wife; for the Child who has been conceived in her is of the Holy Spirit. She will bear a Son; and you shall call His name Jesus, for He will save His people from their sins” (Matthew 1:20, 21 NASB). Aside from the angel's words, there was no reason for Joseph to marry Mary. What would people think; what would the neighbors say? There would be no stigma in sending Mary away. There would be all kinds of stigma in letting her stay. But stay she would because the child conceived in her was of the Holy Spirit.
“And Joseph awoke from his sleep and did as the angel of the Lord commanded him, and took Mary as his wife, but kept her a virgin until she gave birth to a Son; and he called His name Jesus” (24, 25). That's it. And so, as John wrote, the Word who was with God and the Word who was God and the Word through whom everything that is made was made became flesh (1:1, 14). In the Apostolic Creed we confess, “conceived by the Holy Spirit, Born of the Virgin Mary.” In the Nicene Creed, we confess belief in one Lord Jesus Christ who “was incarnate by the Holy Spirit of the Virgin Mary, and was made man.”
In this way, God chose to come to us. Mary's initial disbelief was in her question, “How can this be, since I am a virgin?”(Luke 1:34). God's confirmation was in the angel's reply to her, “For nothing will be impossible with God” (35). These words were repeated by Jesus to His disciples when they wondered how it was that anyone could be saved. “They who heard it said, 'Then who can be saved?' But He said, 'The things that are impossible with people are possible with God'” (18:26, 27). So it is that God did the impossible for us. Jesus, the Word, was born into humanity conceived by the Holy Spirit and born of a virgin. Jesus, the one who knew no sin, became our sin so we could have His righteousness. “He made Him who knew no sin to be sin on our behalf, so that we might become the righteousness of God in Him” (2 Corinthians 5:21). Jesus, the second person of the Trinity, became the perfect man so that He could be the perfect sacrifice to appease the wrath of God. “...being justified as a gift by His grace through the redemption which is in Christ Jesus; whom God displayed publicly as a propitiation in His blood through faith” (Romans 3:24, 25).
Mary knew the Child in her was of the Holy Spirit as the angel had promised her. Joseph also came to know whose Child Mary bore. They both, in obedience that comes by faith, believed the promise God made to them and to us. In this Child, conceived by the Holy Spirit and born of the Virgin Mary, God came to us in human flesh. In flesh He took upon Himself the sin of us all, suffered on the cross through His shed blood, and became in our place the final and prefect sacrifice for the forgiveness of sins.