Luke didn’t make any excuses for starting his story with an angel! We can see why he wrote in his introduction that he had carefully investigated everything. We are naturally sceptical of stories about supernatural things, and angels are usually seen as the inventions of fantasy writers or superstitious people. But Luke knew that he must include the angel, whether or not it seemed believable. God sent a messenger to inform people of what was going to happen.
Two pregnancies are announced; firstly, Zechariah’s wife Elizabeth is going to have a son, despite husband and wife being elderly. As an adult, this son will prepare people for the coming of the Lord (verse 17).
Secondly, six months later, Elizabeth’s relative Mary is told that she is also going to have a son. But this is bewildering to Mary, because she is a virgin. In Mary’s day, remaining a virgin until marriage was expected. Failure to maintain her virginity would have resulted in disgrace or worse. Mary would have been extremely guarded in her interactions with men.
But Mary is told that this child will not have a biological father, but will be conceived by God's power, and will be called the Son of God! Also, as a descendent of King David through his mother, Jesus will be the King who reigns forever.*
The angel also tells Mary that her elderly relative Elizabeth is 6 months pregnant, since “nothing will be impossible with God" (sometimes translated, "No word from God will ever fail", verse 37.) Those words are so important, because these are not typical events. Normally, they are impossible! But if the everlasting Creator God intervened in human history by sending His own Son, what would that look like? Surely the people living at the time would have some pretty incredible stories to tell!