Now all this was done, that it might be fulfilled which was spoken of the Lord by the prophet, saying, Behold, a virgin shall be with child, and shall bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us. (Matthew 1:22-23)
This passage is often read as part of a Christmas sermon celebrating the birth of Jesus Christ. The prophet referred to in the passage is Isaiah, and the quote comes from Isaiah 7:14, which is reproduced in 2 Nephi 17:14.
The recognition that the birth of Christ created a condition of “God with us” is an important concept in understanding why Jesus took on flesh and lived as a man among us.