Joseph realised that the Child was Christ who completed the prophecy of Prophet Isaiah who said: “Behold, the virgin shall conceive and bear a Son, and shall call His name Emmanuel” (Is.7: 14) and “For unto us a Child is born, unto us a Son is given; and the government will be upon His shoulder. And His name will be called Wonderful, Counsellor, Mighty God, Everlasting Father, Prince of Peace. Of the increase of His government and peace there will be no end, upon the throne of David and over His kingdom, to order it and establish it with judgement and justice from that time forward, even forever” (Is.9: 6,7). The angel must have quoted the last part of this verse when he appeared to the Virgin Mary (Lk.1: 31-33)