Mark 1:9-11

9 In those days Jesus came from Nazareth of Galilee and was baptized by John in the Jordan. 10 And when he came up out of the water, immediately he saw the heavens opening and the Spirit descending on him like a dove. 11 And a voice came from heaven, "You are my beloved Son; with you I am well pleased."

One of the most basic, most misunderstood doctrines in scripture is that of the Trinity.  One of the biggest arguments leveled against Trinitarianism is the fact that the word "Trinity" is not actually found in scripture.  The problem with this argument is that it ignores the basic fact that the concept of the Trinity is found throughout scripture.  Genesis 1 says that God said "let us make man in our image", so from the get-go, we know that there's more than one part of the Godhead.

But it takes the Gospels with their early look at the life of Christ to really nail down the concept.  

So first, we have the very first public appearance of the adult Jesus. Here He is.  Except that, unlike today's attempts at public appearances, there were no major marketing campaigns or advertisements.  Jesus simply showed up.  The best indication people had that He was coming, besides being repeatedly told about the coming Messiah in the scriptures, was a bug-eating, hair-wearing, desert-dwelling cousin from the other side of the country.  Not exactly what I'd consider to be a flashy campaign, if there was a campaign at all.  If Jesus did want a campaign, He could have shown up at the temple and start educating people with His massive store of theological knowledge. 

But He didn't.

Jesus showed up for Baptism. Now this is interesting, because basic Christian doctrine says that Christ was sinless, yet Christ submits to Baptism.  He didn't need to have His sins washed away because He didn't have any to wash away.  So why bother?  Well, for the same reason that Jesus offered sacrifices at the Temple as a child: Jesus was baptized so that all of the works of the Law would be fulfilled in Him.  

He humbles Himself to the same level as all of those Jews who acknowledged that the Law’s training wheels weren’t getting it done.  He humiliated Himself – he became the Servant.

Again, keep in mind that Baptism was a very real part of the Jewish faith.  It wasn't just invented in the New Testament - it was a Jewish concept and practice that was later used by God as the sign and seal of the New Covenant.  The concept, again, was that Baptism was required for new converts to Judaism as part of a purification ritual.  For a faithful Jew to be baptized was an act of humiliation, because it acknowledged that they weren't really part of the faith, and that they needed to have their hearts changed at the core - and they responded by being baptized in great numbers.  

And here's the Son of God doing the same thing.  He's humbling Himself, humiliating Himself by allowing Himself to undergo this purification ritual.  He's becoming the lowest servant and fulfilling the works of the Law - all at the same time. 

And then Dad shows up. 

The unique thing about Jesus' baptism - the thing that didn't happen to the other people - was that the Spirit came down in the form of a dove and a voice from heaven proclaimed Jesus “my beloved Son”.  Publicly.  John’s baptisms, again, were attended by large groups of people, and Jesus would have been in the presence of people that would have been witnesses to this.  People who could have, quite literally, hauled Peter and Mark into court and threatened him with their lives if this hadn’t happened.  No one ever did.  The gathered witnesses all saw and heard the same thing.

And it's this same thing that give us our picture, a family portrait of the Trinity.  We have the Father declaring that this baptizee is not only pleasing to Him, but is, in fact, His Son.  To top it off, the Spirit is present "like a dove".  In other words: The Godhead is completely, and physically, present at the Baptism of Christ.  This is the picture of the Trinity we've been in need of since Genesis 1, and it took Jesus' act of humiliating Himself and fulfilling the requirements of the law to give it to us.