“I will be his father, and he will be my son.”
The writer of Hebrews quotes one phrase from a longer passage, because it emphasises the sonship of the Messiah. But he will of course have been aware of the context. The wider passage says:
“The LORD declares to you that the LORD himself will establish a house for you: When your days are over and you rest with your fathers, I will raise up your offspring to succeed you … and I will establish his kingdom. He is the one who will build a house for my name, and I will establish the throne of his kingdom for ever. I will be his father, and he shall be my son.” 2 Samuel 7.11b-14a
This is one of those many examples in the Old Testament where a prophecy has more than one layer of meaning. On the surface it is a promise to David that his son (Solomon) would inherit the kingdom and initiate a dynasty. But ‘for ever’ was not true of Solomon’s descendants; there is another meaning just below the surface, looking forward to a truly permanent kingdom of a different kind, with the Messiah, God’s own Son, ruling not just over Israel, but over a restored creation.
We praise you Jesus because in you God’s promises are fulfilled.