The warning to "beware of the yeast of the Pharisees" is a warning from Jesus to not get so caught up in the trappings of religious behavior that we miss out on the relationship with God that Jesus came to restore.
Who do people say the son of man (Jesus) is? I find it interesting that this is still a core question today. In Jesus time they thought he was some old prophet (or even John the Baptist come back to life); or that he was possibly the advance man for the messiah (see Malachi 4). Even today people question who this Jesus person really is. C.S. Lewis (and others) claim that there are only four options: Jesus is either a legend, a liar, a lunatic or LORD. Some, incredibly, claim that Jesus never really existed and that the Gospels are stuff of legend or another ancient myth. The difficulty here is that there is more than enough evidence to prove Jesus existence. Others say Jesus was just a good teacher (another in a line of those come to show us a better way). But this good teacher put himself on par with God (even claiming to be Son of God -- which makes him as much God as God is). I'm pretty sure if I claimed to be God people would no longer consider me a "good teacher". (They would , quite rightly, have me locked away). Jesus is either the Son of God or he is a liar and not worth consideration. Jesus is a lunatic -- see above. Either he is the Son of God or he is nuts! Or, finally, Jesus is Lord. That is, he is who he claimed to be and deserves our allegiance and devotion. Interesting choices.
We conclude this chapter with some teaching on the Way of the Cross. This is the life Christians are called to live. We are called to deny ourselves (the key to self fulfillment is self denial and service for others). If we try to save or preserve this life we will lose it but if we give it away (lose it) for the sake of Jesus we will gain true life, indeed. The message to our generation is needed here: what does it profit any of us to gain the whole world and lose our Soul? There are a lot of soulless wanderers in my world. Let the material junk go and follow Jesus.