For to which of the angels did He ever say, "You are my Son, today I have begotten You"? And again, "I will be a Father to Him and He shall be a Son to me"? (Hebrews 1:5)
Earlier, we considered the possibility that Jesus, given the uniqueness of His Person, belongs to a category of beings that is intermediate between God and humanity. Specifically, we brought up the question of whether or not Jesus could be some sort of angel. Additionally, we alluded to the fact that, in the epistle to the Hebrews, the author clearly refutes such a proposal by appealing to the reality of Christ's nature. Jesus is not an angel, but is Himself both fully God and fully human; He is indeed God Incarnate.
In order to support such a declaration of Christ's Person, the author begins a series of comparisons between Jesus, as Son of God, and angels. The conclusion of all these comparisons is captured succinctly by a phrase in verse four: Christ is "better than the angels" (Hebrews 1:4). This is the burden of the author in the current passage, as it is the burden of the author throughout the epistle to convince the reader of Christ's supremacy over all. Of all the various attributes that are associated with either Jesus or the angels, the writer begins by affirming Christ's unique nature as a Son.
Importantly, this section (verses 5-14) gives rise to an interesting pattern and focus in the book of Hebrews:
1) The author heavily relies on the authority of the Old Testament
2) The author exclusively quotes the Septuagint, and
3) The author generally ignores the human writer, attributing Scripture to God directly
The second of these points will become relevant, for example, in our discussion of verse 6. With respect to the other two features, the author cites two passages, one from 2 Samuel 7 and the other from Psalm 2, by putting forth the following rhetorical question to consider,
For to which of the angels did He ever say, "You are my Son, today I have begotten You"? And again, "I will be a Father to Him and He shall be a Son to me?"
Narrowing on the author's treatment of Holy Scripture in this question, we see that the mindset is similar to that of the Apostle Paul, who testifies that "All Scripture is inspired by God" (2 Timothy 3:16). So, God is in this verse the subject of "say," and what God said is a direct quotation from the Old Testament. Again, such a concept in Hebrews should now come as no surprise to us, for the writer opens the epistle with the proposition that in times past, God spoke in manifold ways, including through the words He revealed to be recorded in the Scriptures (Hebrews 1:1). Furthermore, God, in His faithfulness and mercy made known to us in these writings much about the One to whom they point(ed) with great anticipation.
With regard to semantic content of the question, the implication is plainly that Jesus is better than the angels as a Son. God never bestowed upon angels the filial relation that Jesus possesses eternally. So, this sonship is a unique relationship that no angel holds, and this is evidenced by God having spoken this beforehand exclusively to/of Jesus, revealing to humanity a crucial aspect of the Messiah and Savior of the world. Jesus, the eternal and only-begotten Son, who is not made, enables us to see the Father through Him as a result of the intimate communion He shares with the Father.
Another facet that is a key to understanding this passage and declaration of Christ's sonship is the notion of inheritance. We read that Christ, who is "heir of all things...has inherited a more excellent name than [the angels]," the name of God's Son (Hebrews 1:2, 4). Christ, the heir of all things has inherited all things by virtue of His sonship and His obedience. We, too, have an inheritance that we yearn to receive, and the Holy Spirit is our divinely-given pledge of promise for this, assuring us that our expectation and desire is not in vain (Ephesians 1:11, 13-14). However, our inheritance is entirely predicated upon Jesus; indeed, without the inheritance of the Son, ours has no foundation. Assuredly, we have "good news" preached to us, as Paul preached to many at Pisidian Antioch, that God has fulfilled the promise "made to the fathers...in that He raised up Jesus, as it is also written in the second psalm, 'You are My Son, today I have begotten You'" (Acts 13:32-33; Psalm 2:7). Having been "raised up from the dead, no longer to return to decay," Jesus, "declared the Son of God with power by the resurrection from the dead," obtained His rightful inheritance (Acts 13:34; Romans 1:4). He wholeheartedly offers and invites us to enter in to His inheritance, that which He guarantees for us by the sacrifice of Himself. And we know that while the "way of life" that we "inherited from [our] forefathers" is "futile," Jesus has redeemed us by His "precious blood," and He has prepared for us "an inheritance which is imperishable and undefiled and will not fade away" (1 Peter 1:18-19, 4). For this we wait, who have been caused by God "to be born again to a living hope through the resurrection of Jesus Christ from the dead," knowing that our inheritance is "reserved in heaven for [us]" (1:3-4). And we are "protected by the power of God through faith for a salvation ready to be revealed at the last time" (1:5).