"And he is the head of the body, the church; he is the beginning and the firstborn from among the dead, so that in everything he might have the supremacy." NIV translation
Now, Paul gives us the answer to the question that we started with: Who's in charge of the church? and we see that it is Jesus Christ. He is supreme over everything and, as we see in Revelation 5, He is the one that owns everything. There are leaders that God has placed in His church for the good of the people but those leaders have to remain humble and subject to the leading of God through the Holy Spirit.
"For God was pleased to have all his fullness dwell in him," NIV translation
Jesus was both fully God and fully man even though this may be hard for us to understand. He was fully God in that He has all of the same characteristics of God (the DNA) yet He came to this earth as a man.
This fact is how we can tell those that truly are God's children from the imposters that have gone out to deceive many. (see 1 John 4).
"and through him to reconcile to himself all things, whether things on earth or things in heaven, by making peace through his blood, shed on the cross." NIV translation
The penalty for sin always has been death and all things were tainted by sin. To have a right relationship with God (be reconciled) required the payment for sin. The Old Testament sacrifices were for individual sins and were only temporary while victory over all sin was accomplished once and for all time by Jesus on the cross.
"Once you were alienated from God and were enemies in your minds because of your evil behavior." NIV translation
Before we come to faith in Jesus Christ, our minds are focused on ourselves and on things of the flesh. This makes us enemies of God because it leads to all kinds of evil actions. This verse reminds us of just how bad sin is. Oh, it may seem like fun for a season but, even then, it is separating us from God and causing us to be His enemies. When we come to Christ, we have a renewal of the mind. This word renewal is like when you remodel a house. You take the old stuff out and replace it with new stuff and that is what happens through the Holy Spirit.