Here's why this is a mystery:
* Mary's pregnancy is miraculous: Mary was a virgin when she conceived Jesus through the Holy Spirit. This is a central mystery of Christian faith.
* Elizabeth's pregnancy is also miraculous: Elizabeth was elderly and had been unable to have children. She conceived John the Baptist through God's intervention.
* The meeting is prophesied: The Bible (Luke 1) describes how Mary's visit to Elizabeth causes John the Baptist to leap in Elizabeth's womb. This is interpreted as a sign of God's plan and the connection between Jesus and John the Baptist.
Why it might be considered a mystery:
* The supernatural nature of both pregnancies is a source of wonder and belief.
* The meeting between Mary and Elizabeth foreshadows the importance of both John the Baptist and Jesus in Christian theology.
If you're interested in exploring this further, you can read the story in Luke 1:26-56 in the Bible.