(Recently Eclectic Orthodoxy (link to the blog here) linked to this post on Facebook. In it the author writes, . . . the Scripture consistently speaks of God as an individual self, and never as a group of individual selves. This post can be taken as a kind of counterexample to that claim. There is a lot more that may be said, too, about its exaggerated biblicism, but those are topics for a possible later post.)
The beginning of the gospel of Mark famously attributes to Isaiah what are actually two distinct prophetic citations, one from Malachi and the other from Deutero-Isaiah:
See, I am sending my messenger ahead of you,
who will prepare your way; (Mal 3.1)
the voice of one crying out in the wilderness:
"Prepare the way of the Lord,
make his paths straight!" (Is 40.3)
There are numerous fascinating aspects to this text, some of which I will try to bring out in the following paragraphs. The conclusion that I give is that these texts have been artfully combined (and suitably modified) so that the result is a kind of Trinitarian reinterpretation of the prophetic discourse in the Old Testament. Mark here is describing a discourse which took place between the Father and the Son, one of which we catch a glimpse in the Old Testament text.
In the first place, as I have had occasion to argue in the past, Mark is here hinting very strongly the doctrine of the incarnation. This "Lord" of which Isaiah spoke is most definitely the LORD; Isaiah is speaking about YHWH himself in 40.3, and this is something easily recognizable when reading the OT text itself. But when John prepares his way through the call for repentance for forgiveness, it is Jesus of Nazareth who shows up (Mark 1.9). This cries out to be interpreted as follows: the way of the LORD is being prepared, and Jesus is the one who shows up; therefore Jesus is the LORD, he is YHWH himself incarnate. He is both divine as LORD and human as coming from Nazareth of Galilee, with a human lineage and heritage.
But there is also a distinction of voices at play here. The Septuagint of Mal 3.1 reads as follows:
ἰδοὺ ἐξαποστέλλω τὸν ἄγγελόν μου, καὶ ἐπιβλέψεται ὁδὸν πρὸ προσώπου μου
Behold, I send my messenger, and he will survey the way before me.
The Masoretic text is the same with the exception that the messenger prepares the way, rather than surveying it. In any case, however, this much is clear: the messenger is the LORD's, and the way is being prepared or surveyed before the LORD. There is a single voice, a single "I" which is speaking here.
In Mark's citation, however, there is a crucial difference. Now there is a "you" whom is being addressed by the LORD. It is still the LORD's messenger who is being sent, and it is still the LORD's way which is being prepared, but now the prophetic text is the speech of an "I" to a different "Thou," who nevertheless is identified with the LORD.
Here we find an incredibly significant modification of the biblical witness to accommodate a new person, a new voice within the godhead. A discourse which had previously contained only one "I" is now rewritten that another may be introduced therein. The way for the Son is being prepared, and the Father is telling him all this ahead of time, in the biblical texts. Mark here is lifting the person of Jesus of Nazareth up the level of divinity, so that he is now one person among many within the single God YHWH. Jesus is not some heavenly being who is yet less than God, since he is the one shows up when it is the way of the LORD that has been prepared. Jesus is the LORD, and yet he is not the Father. Within the godhead there is Son and Father (and Holy Spirit).
Importantly, too, the two prophecies are being ascribed to a single prophet, that the unity of the discourse be recognized. It is one conversation between Father and Son. The significance of the ascription to Isaiah may also be worth noting. I think the point is to identify Jesus with the Suffering Servant of God which plays such a critical role in Isaiah 40-55.
My conclusion, then, is that Mark intends 1.2-3 to be understood as Trinitarian discourse: a discussion of the plan of the Holy Trinity for the salvation of humanity is being dramatized in the artful reworking and reinterpretation of certain critical Old Testament prophetic texts. The Trinity was there, but it was hidden; in Jesus Christ everything is brought to light, and our understanding is changed. The experience of God in Jesus Christ is the epistemological center from which the Old Testament revelation is understood, and even an intentional modification of revelation is given inspired status, because of its origin in what has been revealed.