And it came to pass after I, Nephi, having heard all the words of my father, concerning the things which he saw in a vision, and also the things which he spake by the power of the Holy Ghost, which power he received by faith on the Son of God—and the Son of God was the Messiah who should come—I, Nephi, was desirous also that I might see, and hear, and know of these things, by the power of the Holy Ghost, which is the gift of God unto all those who diligently seek him, as well in times of old as in the time that he should manifest himself unto the children of men. 1 Nephi 10:17

And he said unto me: Behold, the virgin whom thou seest is the mother of the Son of God, after the manner of the flesh.  1 Nephi 11:18

The term "Son of God" - or the idea that God has a son - NEVER showed up until Gabriel announced it to Mary.

And the angel said unto her, Fear not, Mary: for thou hast found favour with God. And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name JESUS. 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: And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end.  Luke 1:30-33

Jesus himself never called himself this, preferring instead the Old Testament term "Son of Man."

Peter used the concept when he confessed to Jesus in Matthew 16:16 - "Thou art the Christ, the Son of the living God."

The reason the Pharisees wanted to kill Jesus was because he never denied he was the Son of God, God Himself, in the flesh.  It was a term utterly foreign to them.