Jesus

"We believe in one Lord Jesus Christ, the only-begotten Son of God, begotten of His Father before all worlds; God of God, Light of Light, very God of very God, begotten not made, being of one substance with the Father by whom all things were made; who for us men and for our salvation, came down from heaven and was incarnate by the Holy Spirit of the Virgin Mary and was made man; and was crucified also for us under Pontius Pilate. He suffered and was buried. And the third day He rose again according to the Scriptures and ascended into heaven and sits at the right hand of the Father.  And He will come again with glory to judge bot the living and the dead, whose kingdom will have no end." - The Nicene Creed, second article

Jesus is not just a religious man or leader, He is very God of very God.  He is our Lord and out God (Jn. 20:28).  We worship Jesus.  And in worshiping Jesus we also worship the Father and the Holy Spirit, as Jesus said, "Whoever has seen me has seen the Father" (Jn. 14:11), and St. Paul writes that, "in [Jesus] all the foulness of God was pleased to dwell" (Col 1:19).

Jesus, then is God.  He is also a man, born of the virgin, Mary.  This man, Jesus, is the incarnation of God, the icon of God.  Because He had no earthly father, He did not inherit the sin of Adam, but only Adam's flesh through Mary.  And being without sin - without hatred toward and enmity of God - He was perfectly obedient to the will of His Father, as He says, "The Son can do nothing of His own accord, but only what He sees the Father doing" (Jn. 5:19).

Obeying the will of the Father, Jesus reconciles sinful man to God by His substitutionary death on the cross.  "As one trespass led to condemnation for all men, so one act of righteousness leads to justification and life for all men" (Ro. 5:18).  Jesus' death on the cross is the death of sin for He who knew no sin became sin for us, and sin was nailed to the tree.  Therefore, by Jesus blood poured out for us, we are made righteous.

Not only so, but this man, Jesus, rose from the dead.  Having obeyed the Father perfectly, He was granted life, eternal life, and was raised up above all creation and seated at the right hand of the Father, having received from His Father the name above every name, Lord.  Ascending to the Father, the Lord Jesus bring us into the presence of the Father, tearing down the wall of hostility that separated us from God.  In Jesus we are united to God.

Jesus is not part of Christian theology, He is Christian theology.  No one knows the Father except the Son and those to whom the Son reveals Him.  In other words, without Jesus, no one knows God or can know Him.  They may know of Him, but they do not know Him.