She will bear a son, and you shall call his name Jesus, for he will save his people from their sins. (Matthew 1:21)
If you think it was difficult for Joseph to trust what the angel said in verse 20 (yesterday), imagine how much more difficult it was for Joseph to trust what the angel said in this verse!
Joseph could get immediate answers and make immediate decisions as a result of what the angel said to him in verse 20. Joseph could go talk to Mary, and Mary could confirm that the angel visited her as well. Joseph could change his mind and not divorce Mary because of what he learned. His faith could be verified pretty quickly.
Even the first phrase in this verse would be verified in a few months. The baby would come, and Joseph would see that he had a son. After he was born, he could name him Jesus. This could all be very quickly verified, within a matter of months.
But what did the angel mean by the words, he will save his people from their sins? No person could do that, could he? The Jewish people had a temple and a sacrificial system. Sacrifices had to be offered daily, weekly and yearly for sins. These sacrifices didn’t take care of sins completely, because they had to be continually offered. God never said anything about a human sacrifice, did he? How could a person save if his people from their sins?
One man, Adam, brought sin into the world. Could one man save from sin? Yes! Joseph would not immediately fathom the meaning of the angel’s words. Through the next 33 years, Joseph would come to learn what the angel meant. His son would live a sinless life, die a sacrificial death, and rise again to defeat sin and death. Thank God for giving Joseph enough faith to believe even though he could not see and understand.