The name Jesus, because he will save his people from their sins

 Matthew 1:18-25

But after he had considered this, an angel of the Lord appeared to him in a dream and said, “Joseph son of David, do not be afraid to take Mary home as your wife, because what is conceived in her is from the Holy Spirit.  She will give birth to a son, and you are to give him the name Jesus, because he will save his people from their sins.”

All this took place to fulfill what the Lord had said through the prophet: “The virgin will conceive and give birth to a son, and they will call him Immanuel” (which means “God with us”).

When Joseph woke up, he did what the angel of the Lord had commanded him and took Mary home as his wife. But he did not consummate their marriage until she gave birth to a son. And he gave him the name Jesus.

Luke's fuller account of the birth of Jesus focuses on Mary: Matthew's on Joseph. Joseph is a descendant of David and it is his naming/adoption of Jesus (who was not his son) that makes Jesus officially a 'son of David' as the Scriptures require. In verse 23 Matthew gives us the first of five Old Testament quotations used in chapters 1 and 2 to show what Jesus fulfills all that the prophets foretold.

Here Isaish'w words take on a new dimension: in Jesus God is actually here in our world with us (the meaning of the name 'Immanuel')

Why?

From the point of view of Joseph, he won't marry Mary the Holy Spirit told him, so Joseph did as God said, it is fulfilled what the Lord has said through the prophet Isaiah.

Jesus's name is meant to save his people from their sins. His people follow the word of Jesus.

Immanuel means God with us, God actually comes to the world with us.

God's word never fails. God's promises will be done. Trust in the Lord's word.

How?

Jesus is the Lord and Jesus came to save us from our sins.