Matthew 3:16

Apart from the fact that Jesus confessed no personal sin as he was being baptized, the process of baptism probably did not vary from the several which John had already performed. As Jesus came up out of the water however, all heaven broke loose.
The conjunction δὲ functions transitionally to introduce a new development and serves to return us to the main focus from the conversation initiated by the δὲ in 3:14.1 The temporal adverbial participle βαπτισθεὶς qualifies the main verb ἀνέβη as does the temporal adverb εὐθὺς. The combined double qualifier together with the culminative aorist ἀνέβη serves to picture the departure of Jesus from the water as completed. There could be some question as to whether the timing is in reference to Jesus standing up after going under the water or if it refers to him walking out of the water after the entire baptismal ceremony. The temporal adjectival adverb εὐθὺς seems to indicate that the descent of the Spirit began to happen as soon as Jesus had finished standing up to complete the cycle of baptism.
The adjunctive καὶ2 introduces the exclamatory interjection ἰδοὺ. This construction is repeated in v17. In fact the phrase is used 28 times3 in the book of Matthew. In almost every instance it serves to draw the reader's attention to a new and often unexpected development.4
Luke 3:21 reveals that Jesus was praying as he came out of the water and it was as he prayed that the heavens were opened (ἠνεῴχθησαν) meaning that visibly either the crowd, John, Jesus or a mixture of any of the three were enabled to see into the realm of glory. Else it could graphically indicate that the interaction between God and Mankind was about to take on a completely new venue in the person of Jesus (cf. Hebrews 1:1-2). From a visual standpoint it is nearly impossible to determine what was seen. The dramatic aorist however probably points Matthew's Jewish readers to the opening of Ezekiel in which the heavens are opened to the prophet in preparation for his ministry.
As discussed in Textual Variants5 the pronoun αὐτῷ is uncertain in the text. Initially it is not entirely clear if everyone else was able to see what Jesus saw. The HCSB translates the questionable αὐτῷ as a dative of advantage "...the heavens suddenly opened for Him..." This is also done in parenthesis in the New American Bible. The KJV and ESV, NRSV, Darby (1890) translate it as a dative of indirect object "...the heavens were opened unto / to him..." The NIV and NASB95 both opt to discard the personal pronoun omitting it altogether from the translation. If the pronoun belongs in the text however, it is likely functioning as a dative of advantage. While the opening of the heavens would be for the benefit of Jesus as indicated by the usage of the personal pronoun σὺ in the divine pronouncement of Mark 1:11 and Luke 3:22;6 that would permit the others present, at least John (cf John 3:13) to see and hear both the audible (v17) and visual event which Jesus saw and heard.
Jesus saw (εἶδεν) [τὸ] πνεῦμα [τοῦ] θεοῦ. The genitive of description indicates that the Spirit in question is God's Spirit in particular. The days of Jesus and John the Baptist were not marked by a deep sense of the Spirit's involvement. Keener expresses the Jewish mindset:
"Many believed that the Spirit was no longer available in their time; others believed that the Spirit simply did not work as forcefully as in the days of the prophets, until the time of the end. That the Spirit comes on Jesus indicates the inauguration of the messianic era and marks Jesus out as the Spirit-bearer and hence Messiah (3:11)."7
1Compare the introduction to 3:14 above p. 14.
2Young, Intermediate New Testament Greek. p. 188.
3Matt. 2:9; 3:16f; 4:11; 7:4; 8:2, 24, 29, 32, 34; 9:2f, 10, 20; 12:10, 41f; 15:22; 17:3, 5; 19:16; 20:30; 26:51; 27:51; 28:2, 7, 9, 20
4Young, Intermediate New Testament Greek. p. 199.
5See above p. 7.
6See discussion on οὗτός at verse 17 p. 29.
7Craig S. Keener and InterVarsity Press, The IVP Bible Background Commentary : New Testament. (Downers Grove, Ill.: InterVarsity Press, 1993), Mt 3:16.
8Marvin Richardson Vincent. Word Studies in the New Testament. (Bellingham, WA: Logos Research Systems, Inc., 2002), Matthew 1:26.
9Donald A. Hagner. vol. 33A, Word Biblical Commentary : Matthew 1-13, Word Biblical Commentary. (Dallas: Word, Incorporated, 2002), p. 58.