“Look!” The Greek word is idou (#2400 ἰδού), and it is used to get our attention. See commentary on Matthew 1:20 (“Look!).
“This is.” What God said at Jesus’ baptism is recorded in Matthew, Mark, and Luke, and in Mark and Luke, God speaks directly to Jesus, saying, “You are my beloved Son.” Here in Matthew, the text says, “This is” my beloved Son. While it is possible that God made more than one statement about His Son, one being “You are” and one being “This is,” that is not likely. The greater possibility is that Mark and Luke recorded what the voice from heaven actually said, while Matthew recorded what the voice fully intended: that we the audience be included in the knowledge that Jesus is the Son of God. Thus, when Matthew, Mark, and Luke are put together and understood as communicating God’s heart to mankind, we see that Jesus got clear confirmation that he was the Son of God, and God intends for us to have that understanding too. That Jesus is the Son of God is not a message just for Jesus. It is for the world to know. It seems likely from John 1:34 that the audience, including John the Baptist, heard the voice from heaven as well as Jesus did.
Due to the pressure to harmonize scriptures so that the same record reads the same way in different Gospels, the Greek manuscript “D” from the 5th century, and some Syriac (Aramaic) manuscripts, have Matthew read “You are my beloved Son,” like Mark and Luke do. Thankfully, that harmonization was copied into so few manuscripts that it does not show up in any well-known version of the Bible. In support of the reading “This is my beloved Son” is not only almost every known Greek manuscript and some Syriac manuscripts, but also the Shem Tov Hebrew manuscript of Matthew. The Shem Tov Hebrew manuscript is believed to be a lineal descendant of the original manuscript of Matthew that the Apostle Matthew wrote in Hebrew [for more on the Shem Tov Hebrew Manuscript of Matthew see commentary on Matthew 3:3].
One last reason to believe that “this is” was the original reading of Matthew 3:17 is that there would be lots of pressure to change “this is” to “you are,” but no pressure to change “you are” to “this is.” That makes “This is” what textual scholars call “the more difficult reading,” which in most cases is the original reading because scribes tended to make the text easier to read and understand rather than harder to read and understand. Given all the evidence, “This is my beloved Son” can be seen to be the original reading of Matthew 3:17. [For more on the harmonization of Scripture, and how it has affected translations, see commentary on Luke 11:2, “Father”].