When was Jesus first referred to as the Son of God in the Old Testament?

Answer:

"I will surely tell of the decree of the LORD: He said to Me, 'You are My Son, Today I have begotten You. Ask of Me, and I will surely give the nations as Your inheritance, and the very ends of the earth as Your possession. You shall break them with a rod of iron, You shall shatter them like earthenware.' Now therefore, O kings, show discernment; Take warning, O judges of the earth. Worship the LORD with reverence And rejoice with trembling. Do homage to the Son, that He not become angry, and you perish in the way, for His wrath may soon be kindled. How blessed are all who take refuge in Him!" (Psalms 2:7-12).

This prophecy is quoted in Acts 13:33; Hebrews 1:5; and Hebrews 5:5 and applied to Jesus.

"For a child will be born to us, a son will be given to us; and the government will rest on His shoulders; and His name will be called Wonderful Counselor, Mighty God, Eternal Father, Prince of Peace. There will be no end to the increase of His government or of peace, on the throne of David and over his kingdom, to establish it and to uphold it with justice and righteousness from then on and forevermore. The zeal of the LORD of hosts will accomplish this" (Isaiah 9:6-7).

An angel alludes to this prophecy and applies it to Jesus. "The angel answered and said to her, 'The Holy Spirit will come upon you, and the power of the Most High will overshadow you; and for that reason the holy Child shall be called the Son of God'" (Luke 1:35).

"When Israel was a youth I loved him, and out of Egypt I called My son" (Hosea 11:1).

This is applied to Jesus in Matthew 2:5.

See also Subtle References to the Trinity of God.