The fundamental tenet of Christianity is that God created man with a view toward inviting man to partake of salvation, guaranteeing it by way of the life and death of Jesus Christ. In Catholic tradition, salvation is implied by the Creation. The catechism says that God made us "to know Him, to love Him, and to serve him in this world, and to be happy with Him forever in the next" (Kinkead 29). Being happy in the next world means saving one's soul, and to accomplish that "we must worship God by faith, hope, and charity; that is, we must believe in Him, hope in Him, and love Him with all our heart" (Kinkead 310).
The lesson of the Old Testament is that man abdicated salvation by "falling into sin," with the story of Adam and Eve illustrating the point. That is where Christ enters the picture:
God did not abandon man after he fell into sin, but promised him a Redeemer, who was to satisfy for man's sin and reopen to him the gates of Heaven (Kinkead 67).
Hence the Passion and the Atonement, also called the Redemption, which represents God's covenant with mankind that mankind's sin has been expiated by Jesus' death. It is sufficient to reinvite mankind to salvation because Jesus is not only a man but also the Son of God and indeed is one with God Himself.
Now that sounds like very good news for mankind, but from the time salvation doctrine was voiced by the emerging church, it was controversial. A core issue was the deity of Christ. Some early Christians argued that Jesus was special but, finally, a special man merely. That Arian heresy was crushed by the Council of Nicaea in AD 325. Jesus is not special but divine, the Son of God, and being God, coequal with God the Father, "consubstantial . . . not in the manner of bodies [or] . . . of any other hypostasis or essence, but of the Father" (Eusebius 339). Father and Son were also consubstantial with the Holy Spirit (though Trinitarian doctrine posed its own vexations). The present...