While Christianity is a Monotheistic religion—claiming, like Judaism and Islam, that there is a single God—the single God that Christians worship is believed to be (by most Christians) a Trinity rather than just a single person. The theology of the Trinity is complicated and relies on philosophical explanations of the difference between natures, persons, essences, and so on. However, it can be summed up by saying that God is present in three persons—the Father, Son, and the Holy Spirit. The vast majority of Christians believe in a triune God, and the second person of the Trinity, the Son, is Jesus.
The fact that Christians believe Jesus is God, that he is God become man, makes him central to every part of Christian teaching and belief. Jesus being the Christ, fully God and fully man, leads to many other beliefs in Christian teaching, such as the atonement for the sins of man on the cross (why Christ is called the sacrificial lamb), the death and resurrection of Christ (allowing Christians who also believe to be raised at the final judgment), and Christ’s departing gift of the Holy Spirit (the third person of the Trinity, also fully God in Christian teaching) to in-dwell among the disciples.
Every facet of Christian teaching traces back to Christ, whether a lesson from him about how to live on Earth or something theological from the Christian understanding of the incarnation. To put it simply, Christ is who every Christian puts their faith and hope into, because without Christ being truly God, and truly having died and risen again, the Christian faith cannot exist. Saint Paul sums up the central importance of Christ to the Christian faith in the fifteenth chapter of his First Epistle to the Corinthians when he writes,
Now if Christ be preached that he rose from the dead, how say some among you that there is no resurrection of the dead? But if there be no resurrection of the dead, then is Christ not risen: And if Christ be not risen, then is our preaching vain, and your faith is also vain. Yea, and we are found false witnesses of God; because we have testified of God that he raised up Christ: whom he raised not up, if so be that the dead rise not. For if the dead rise not, then is not Christ raised: And if Christ be not raised, your faith is vain; ye are yet in your sins. Then they also which are fallen asleep in Christ are perished. If in this life only we have hope in Christ, we are of all men most miserable. (1 Corinthians 15:12–18, KJV)
If Christ is not God, if he didn’t rise from the dead, the faith of Christians is in vain. Christ is the foundation and fulfillment of the entire Christian faith—which is why Jesus is so important to them.