And behold, the heavens were open to Him. Matthew 3:16

 When I ended my look into Matthew's gospel, Jesus had come to John (who was baptizing in the Jordan) so that John would baptize Him.

“But John tried to talk him out of it. “I am the one who needs to be baptized by you,” he said, “so why are you coming to me?”
Matthew 3:14 NLT)

How long did that conversation last?

We may never know, nor will we know what arguments John was trying to use against Jesus being baptized. It seems rather obvious that John recognized that the Holy Spirit and the “fire” associated with that baptism had more value. Pay attention to Jesus' response.

“Let it be so now,” Jesus replied. “It is fitting for us to fulfill all righteousness in this way.” Then John permitted Him.
Matthew 3:15 BSB)

Here is what we typically see.

“... Allow it now, for it is becoming to us to fulfill all righteousness...” MKJV

I searched, in multiple ways, for any law that would impose full immersion baptism; I found none. The Jews had ceremonial washings that were associated with the law, but none of those required a full immersion. Mikvehs were NOT required but carried the idea of a rebirth and were completely thought of as a ceremonial event. To participate in a Mikveh cleansing, the person would get naked, squat in the pool so that they were completely underwater and stay there for a couple of seconds, and then arise and get out. At this point, you are considered reborn. This is NOT what Jesus was here to do.

And there was Moses washing Aaron and his sons in preparation for temple service.

Exodus 29:1-4 BSB  “Now this is what you are to do to consecrate Aaron and his sons to serve Me as priests: Take a young bull and two rams without blemish,  2)  along with unleavened bread, unleavened cakes mixed with oil, and unleavened wafers anointed with oil. Make them out of fine wheat flour,  3)  put them in a basket, and present them in the basket, along with the bull and the two rams.  4)  Then, present Aaron and his sons at the entrance to the Tent of Meeting and wash them with water.

With no real answers, we will move on.

“When He had been baptized, Jesus came up immediately from the water; and behold, the heavens were opened to Him, and He saw the Spirit of God descending like a dove and alighting upon Him.”
Matthew 3:16 NKJV)

“ Jesus came up immediately from the water,”

Coming up out of the water is what anyone would do. What is my thought and motivation for centering on this statement? Many years ago, there was a push in a recovery group I was associated with to go to attend a weekend retreat/convention with an organization called Cleansing Streams. One of the men from our group, who was enamored with himself, was one of the first to stand up when the opportunity arose to get publicly baptized in the hotel pool. Well, they dunked him and seconds later he emerged from the water as though he exploded and he was shouting “Praise God” and “Hallelujah.” while swinging his arms above his head in some forced act of praise. Nothing changed in his life as he continued in his normal egotistical patterns. Is this what we see with Jesus?

No, but He went up straightway.

STRAIGHTWAY, adv. [straight and way.] Immediately, without loss of time, without delay.

There was no flashy show, but He was a man determined.

“...as Jesus came up out of the water, the heavens were opened and he saw the Spirit of God descending like a dove and settling on him.” Matthew 3:16 NLT)

Opened is the Greek word anoigō. This word was used in John 1:51, where Jesus, who had responded to Nathaniel's sarcastic remark as though He was right there when he said it, “Can any good thing come out of Nazareth?”

“...Most assuredly, I say to you, hereafter you shall see heaven open, and the angels of God ascending and descending upon the Son of Man."
John 1:51 NKJV)

We can look into a sky filled with clouds and, in a moment's time, witness them part exposing the heavens. At that moment, you are seeing the heavens open.

Did you see the angels ascending and descending?

Tell the truth, you did not. And yet, we all sense that this simple phrase implies something more, something that I cannot explain.

“... and he saw the Spirit of God descending like a dove and settling on him.”

Through this opening in the heavens, the Spirit of God descended gently, almost without notice, much like doves do.

Did anyone else, like John the Baptizer, see this opening?

Apparently not.

Then how do we know the heavens opened, and the Holy Spirit came down upon Him? 

Upon questioning by the disciples, I can only assume that Jesus told them what happened. 

Fast forward a bit, and we will see the disciples, under instructions from Jesus, meeting in “the upper room.” There were at least 120 regulars at this time, and I do not think they saw the heavens open as they gathered there. Still, enough of them saw the Holy Spirit, the thing they were waiting for, lite upon them all like large flickers of a candle, and enough of them witnessed this to validate what transpired.

Then how do we know the heavens opened, and the Holy Spirit came down upon Him? 

Upon questioning by the disciples, I can only assume that Jesus told them what happened. 

Fast forward a bit, and we will see the disciples, under instructions from Jesus, meeting in “the upper room.” There were at least 120 regulars, and I do not think they saw the heavens open as they gathered there. Still, enough of them saw the Holy Spirit, the thing they were waiting for, lite upon them all like large flickers of a candle, and enough of them witnessed this to validate what transpired.

How else do we know that something had changed the lives of the disciples after that moment in time? 

Evidence; power; boldness; speaking in other tongues – fortunately, some spoke unfamiliar languages, and others heard the sounds outside on the street.

Scriptural examples:

Timothy 3:16 BSB “By common confession, the mystery of godliness is great: He appeared in the flesh, was vindicated by the Spirit, was seen by angels, was proclaimed among the nations, was believed in throughout the world, was taken up in glory.

The question is, what method did He choose to appear in the flesh.

John 1:14 NKJV  “And the Word became flesh and dwelt among us, and we beheld His glory, the glory as of the only begotten of the Father, full of grace and truth.”

John's gospel merely says that the Word became flesh. Again we are not told how that came to be.

Hebrews 2:9 NASB "But we do see Him who was made for a little while lower than the angels, namely, Jesus, because of the suffering of death crowned with glory and honor, so that by the grace of God He might taste death for everyone."

And the writer of the letter to the Hebrews tells us that He was made. It is hard to read those words without seeing that decision forced upon Jesus, which it was not.

“And remember that the heavenly Father to whom you pray has no favorites. He will judge or reward you according to what you do. So you must live in reverent fear of him during your time here as “temporary residents.” For you know that God paid a ransom to save you from the empty life you inherited from your ancestors. And it was not paid with mere gold or silver, which lose their value. It was the precious blood of Christ, the sinless, spotless Lamb of God. God chose him as your ransom long before the world began, but now in these last days he has been revealed for your sake. Through Christ you have come to trust in God. And you have placed your faith and hope in God because he raised Christ from the dead and gave him great glory. You were cleansed from your sins when you obeyed the truth, so now you must show sincere love to each other as brothers and sisters. Love each other deeply with all your heart. For you have been born again, but not to a life that will quickly end. Your new life will last forever because it comes from the eternal, living word of God.” 1 Peter 1:17-23 NLT)

I have, with purpose, read through the entire Bible. I have also listened to the Bible at least five times over the last few years. That being said, I have read and understood that Jesus voluntarily submitted Himself to be a man, pay the price for us, and now sits at the Father's right hand as our high priest and as a man, our mediator. All the passages I posted here speak to these things, but not as directly as I said. Because I read it at some point, I know it's there in the Bible and have every confidence in my statements about Him voluntarily doing this for us. I also have confidence in the fact that Jesus chose to remain human.