Who told Joseph that Jesus would save people from their sins?

Answer 1

This was the angel Gabriel. The actual quote is in Matthew 1:21 but the angel is not identified, whereas the parallel account in Luke identifies the angel as Gabriel in Luke 1:27:-

Mat 1:18-25 KJV Now the birth of Jesus Christ was on this wise: When as his mother Mary wasespoused to Joseph, before they came together, she was found with child of the Holy Ghost. [v. 19] Then Joseph her husband, being a just man, and not willing to make her a publick example, was minded to put her away privily. [v. 20] But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife: for that which is conceived in her is of the Holy Ghost. [v. 21] And she shall bring forth a son, and thou shalt call his name JESUS: for he shall save his people from their sins. [v. 22] Now all this was done, that it might be fulfilled which was spoken of the Lord by the prophet, saying, [v. 23] 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. [v. 24] Then Joseph being raised from sleep did as the angel of the Lord had bidden him, and took unto him his wife: [v. 25] And knew her not till she had brought forth her firstborn son: and he called his name JESUS.

Luk 1:26-35 KJV And in the sixth month [ie of Elizabeth's pregnancy] was sent from God unto a city of Galilee, named Nazareth, [v. 27] To a virgin the angel Gabriel espoused to a man whose name was Joseph, of the house of David; and the virgin's name wasMary. [v. 28] And the angel came in unto her, and said, Hail, thou that art highly favoured, the Lord is with thee: blessed art thou among women. [v. 29] And when she saw him, she was troubled at his saying, and cast in her mind what manner of salutation this should be. [v. 30] And the angel said unto her, Fear not, Mary: for thou hast found favour with God. [v. 31] And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name JESUS. [v. 32] He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David: [v. 33] And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end. [v. 34] Then said Mary unto the angel, How shall this be, seeing I know not a man? [v. 35] And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.