What does it mean that Jesus was declared to be the only begotten Son of God?

Les explains that verse 4 says Jesus was “declared to be the Son of God” by power, according to the Spirit of holiness, by the resurrection from the dead. He emphasizes that it was the power of God that raised Him from the dead, and that this declares Him then to be the only begotten Son of God. Les teaches that the resurrection stands at the heart of the gospel. He says that if a person does not have the power of the resurrection as the basis of salvation, they have none, because it all rests on the fact that God raised Him from the dead. The context also notes that Jesus was made of the seed of David according to the flesh, and Mary was of the seed of David, as was Joseph.