The heart of Christianity is the death and resurrection of Jesus. It is through the death and resurrection that we are saved. In some mysterious way that transcends our ability to explain, this death of a man saves those who will believe in Jesus.
There are many ways of understanding this. It is a reconciliation of God and humanity. It is the justification of humanity, in which those who believe in Jesus are declared righteous before God, not on the basis of their merit or goodness in any way, but because Jesus has taken their punishment and their separation from God on their behalf. It is a propitiation in which the wrath of God is averted. It is an expiation in which those who believe are cleansed of sin. He is our representative, standing in the place of humanity as God the man and we are saved "in Christ". He is our substitute, dying on the cross in our place, taking our punishment. It is our adoption, in which we are adopted as the children of God. It is our regeneration or rebirth in which we die in Christ and are raised to new life in him, born again from heaven above to live with God forever. It is our sanctification in which we are declare pure despite being impure, and so called to live our of this purity a life of sanctification. It is our recreation, where we are inwardly born anew as a new creation, created in Christ to do good works. It is the conquest of God over evil, over sin, over law which reveals sin and over death. It is our resurrection, Jesus' resurrection from the dead the first fruits of the glorious harvest of the resurrection of the faithful to everlasting life. It is our entry into the Kingdom, the realm of the King. It is the moment of our incorporation into the community of God, the church visible and invisible, the people of God from creation to the present, his body, the temple of the Spirit. It is life, the overcoming of death. It is the fusion of heaven and earth, the restoration of God's ideals, Eden restored. It is atonement, the coming together of God and humanity, the covering of sin. It is our example, Jesus' sacrifice, suffering and death, our example of supreme living and love. We are to emulate this servanthood, laying down our life for the mission of God to see every person come to know his love and be reconciled to him; and to see his world become what he dreams it could be. I could go on and on!
Over history theologians have grappled with this issue, what does the cross mean, what is it's core. Some go for penal-substitution; some for reconciliation; some for representation; some propitiation; some justification... It is fair enough to grapple with this and to work through the meaning of the cross. It is good to look at it from every perspective, for it is like a diamond that has many faces and it is truly beautiful. But let's not fall into conflict about it; where people genuinely acknowledge that this is the only means of salvation for all humanity, they acknowledge Christ as saviour through the cross and resurrection, they are saved.
For me, this is worth dying for!