in the Old Testament, this title was used of kings
the phrase 'son of' means 'reflecting the nature of/like'
kings were thought to have been adopted by god at their accession. they were meant to reflect god's justice and mercy in their rule
by the time of Jesus, it was linked with the title of 'messiah', which also included the idea of being chosen by god to rule on his behalf
in the New Testament, the title indicated Jesus' unique divine authority:
at the beginning of mark's gospel, Jesus is referred to as the son of god and he is said to be 'my beloved son' in the visions at his baptism and transfiguration
quote to show Jesus as son of god
'all things have been committed to me by my father. no one knows the son except the father, and no one knows the father except the son'-Matthew11:27