Adam Gopnik in The New Yorker:
When we meet Jesus of Nazareth at the beginning of the Gospel of Mark, almost surely the oldest of the four, he’s a full-grown man. He comes down from Galilee, meets John, an ascetic desert hermit who lives on locusts and wild honey, and is baptized by him in the River Jordan. If one thing seems nearly certain to the people who read and study the Gospels for a living, it’s that this really happened: John the Baptizer—as some like to call him, to give a better sense of the original Greek’s flat-footed active form—baptized Jesus. They believe it because it seems so unlikely, so at odds with the idea that Jesus always played the star in his own show: why would anyone have said it if it weren’t true? This curious criterion governs historical criticism of Gospel texts: the more improbable or “difficult” an episode or remark is, the likelier it is to be a true record, on the assumption that you would edit out all the weird stuff if you could, and keep it in only because the tradition is so strong that it can’t plausibly be excluded. If Jesus says something nice, then someone is probably saying it for him; if he says something nasty, then probably he really did.
So then, the scholars argue, the author of Mark, whoever he was—the familiar names conventionally attached to each Gospel come later*—added the famous statement of divine favor, descending directly from the heavens as they opened. But what does the voice say? In Mark, the voice says, “You are my Son, whom I love; with you I am well pleased,” seeming to inform a Jesus who doesn’t yet know that this is so. But some early versions of Luke have the voice quoting Psalm 2: “You are my Son; today I have begotten you.” Only in Matthew does it announce Jesus’ divinity to the world as though it were an ancient, fixed agreement, not a new act.