How many songs or sermons have you heard implying that Mary (or Joseph) knew the true purpose of Jesus’ birth? There’s a whole song built around the phrase “Mary Knew” — implying she knew that Jesus was God, that he died to redeem all humanity from their sins, and so forth. In the song “Then Came the Morning,” it says Mary knew Jesus would rise again. There’s a song called “Strange Way to Save the World,” written from the perspective of Joseph, indicating by its very title that Joseph knew that the Nativity was connected with the entire human race.
While we can look back in hindsight and see God’s entire redemptive plan in perspective, such assumptions of full knowledge on Mary and Joseph’s part would be anachronistic and foreign to their cultural perspective on the Messiah. Consider the angel’s message at the Annunciation:
And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name JESUS. He shall be great, and shall be called the Son of the Highest, and the Lord God shall give unto him the throne of his father David. And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end. (Luke 1:31-33)
Now, compare this with the prophet Jeremiah:
And I will gather the remnant of my flock out of all countries whither I have driven them, and will bring them again to their folds…Behold the days come, saith the Lord, that I will raise unto David a righteous Branch, and a King shall reign and prosper, and shall execute judgment and justice in the earth. In his days Judah shall be saved, and Israel shall dwell safely, and this is his name whereby he shall be called, THE LORD OUR RIGHTEOUSNESS. (Jeremiah 23:3ff)
Put yourself in Mary’s position: What is her first and most natural idea of this baby’s purpose going to be? Immediately, she imagines a great ruler, an earthly Messiah who will save the people of Israel. I emphasize “people of Israel” to point out the contrast with what the song describes as “saving the world.” The rest of the world wouldn’t really have factored into Mary and Joseph’s concept of the Messiah. They had specifically Jewish prophecies in mind. The Magnificat says, “He remembering his mercy hath holpen his servant Israel : as he promised to our forefathers, Abraham and his seed for ever.”
This was Zechariah’s reaction too:
Blessed be the Lord God of Israel, for he hath visited and redeemed his people, and hath raised up an horn of salvation for us in the house of his servant David, as he spake by the mouth of his holy prophets, which have been since the world began, that we should be saved from our enemies, and from the hand of all that hate us….That he would grant unto us, that we being delivered out of the hand of our enemies might serve him without fear, in holiness and righteousness before him, all the days of our life. (Luke 1:68-75)
“But wait a minute!” you say. What about all the other prophecies about a suffering Messiah—“By his stripes we are healed,” and all that?
The Jews knew all about those prophecies too. That was why they developed the idea that there would be not one, but two Messiahs—the ruling one and the suffering one. It’s entirely plausible that this tradition dates back to at or before the time of Christ. We certainly find it in ancient Talmudic commentaries. This passage from a commentary on the book of Zechariah summarizes:
The rabbis taught: The Messiah b. David, who (as we hope) will appear in the near future, the Holy One, blessed be He, will say to him: Ask something of me and I will give it to thee, as it is written [Ps. ii. 7-8]: “I will announce the decree . . . Ask it of me, and I will give,” etc. But as the Messiah b. David will have seen that the Messiah b. Joseph who preceded him was killed, he will say before the Lord: Lord of the Universe, I will ask nothing of Thee but life. And the Lord will answer: This was prophesied already for thee by thy father David [Ps. xxi. 5]: “Life hath he asked of thee, thou gavest it to him.”
So, this concept certainly wasn’t alien to the Jews. This means God couldn’t be accused of playing a trick on them by “setting them up” for a ruling Messiah, only to send a different kind of Messiah, one they couldn’t have possibly recognized at all. However, it is worth noting that at the time of the Nativity, Jesus’ parents and relatives would not have been unreasonable in seeing him as a future deliverer-king-ruler, with a kingdom of this world. It would in fact have been their first thought. And even after Jesus’ resurrection, we see his disciples still asking whether he would restore the kingdom to Israel.
Certainly, Mary knew that Jesus was the Son of God. But the idea that Jesus was God was so staggering that Jesus had to repeat it over and over again before it fully sank in even for his closest friends. (In fact, it is entirely possible that they only fully grasped the deity of Christ after His resurrection and ascension. But Peter definitely “gets it” by the time we reach Pentecost.) And the idea that somehow, this baby who was prophesied to be a mighty ruler would also atone for the sins of the whole world… Well, that was why John the Baptist’s pronouncement at the beginning of Jesus’ ministry was so astonishing: “Behold the Lamb of God, behold Him who taketh away the sins of the world!” How could this be? John himself may not have fully understood the magnitude of his own prophecy.
(I can’t take credit for gathering all this info myself. I relied heavily on this blog post, which covers the topic even more extensively than I did here. I’d encourage you to read it—it’s fascinating stuff.)