What is spoken to Ahaz then is to be thus understood. This Child, that shall be born of a Virgin of the house of David, shall now be called Emmanuel, that is, God with us, because the events (perhaps delivery from the two hostile kings) will make it appear that you have God present with you. But after He shall be called Jesus, that is, Saviour, because He shall save the whole human race. Wonder not, therefore, O house of David, at the newness of this thing, that a Virgin should bring forth a God, seeing He has so great might that though yet to be born after a long while, He delivers you now when you call upon Him.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).