Luke 3:21

Now when all the people were baptized, it came to pass, that Jesus also being baptized, and praying, the heaven was opened,

Now when all the people were baptized,.... That came from several parts to John for this purpose, even as many as he judged to be proper subjects of that ordinance, as many of the common people, publicans, soldiers, &c.

it came to pass that Jesus also being baptized; of John in Jordan, he coming from Galilee thither on that account:

and praying; after he was baptized, for the coming down of the Spirit upon him, as man, to anoint, and qualify him for his office he was now about to enter on publicly: and for success in it, and for a testimony from heaven, that he was the Son of God, and true Messiah:

the heaven was opened;

See Gill on "Matthew 3:16".