First, there seems to be no plausible explanation for true free will without the supernatural. Although it's therefore tempting to argue that there is no free will, it's obvious from everyday experience that there is. (Studies which purport to show that our decisions are made before we consciously make them only demonstrate an interesting point about how our consciousness works, not about whether we are ultimately free to make decisions. The experiments prove nothing about whatever began the build-up to the decision.)
If there is a supernatural being behind every human, it stands to reason that there is an even greater supernatural being of which the smaller beings are a part, or are split off of somehow. This is exactly what Christianity teaches. What is Christ if not part of the Greater Spirit (God) combined with a human one? In fact, the very notion of the Trinity reflects the metaphysics which makes most sense based on simple observation. God, or the Spirit, is united but at the same time has different manifestations. God is God the Father, the Creator, while at the same time the Son, the human. There is also the pervasive Holy Spirit which fills all of Creation.
It's clear that the Old Testament began with a people who were prepared only for a tribal Deity, not the real Monotheism of later Christianity. In the Old testament, God acts like a polytheistic God - angry, jealous, loving, vengeful... in other words, human, but bigger. He is a nations's God, rather than the only God. He says, "worship no other Gods before me," not "I am the only God who exists."
Judaism paved the way for Christ's message. Jesus was himself both God and Jew, and so formed a new covenant, this time not with one nation, but with anyone willing to accept Him. At this point, the people had been weaned off of primitive, pagan-like sacrifices and ritual laws of kashrut and the like were no longer necessary. Christ showed people how to interact directly with God without needing such primitive intermediaries.
Some claim that Jesus did not exist. This is almost certainly untrue. He is a recent enough figure and he shows up not just in the four gospels, but in extra-Biblical documents like Josephus and the Talmud. Some point out that the gospels contradict each other in places, for example with the details in the story of His resurrection. I agree that there are contradictions, but that this is to be expected when hearing a story from four different sources. I don't believe that the NT (or the OT) were dictated from God, but it's clear they were inspired from the comfort they give to millions.
The OT paved the way for Jesus Christ. There are many prophets before Jesus, but the OT also speaks of the Messiah. If the Old Testament has any truth to it, there's no way the Second Temple could have been destroyed with no Messiah coming for over two thousand years. Even when the nation of Israel was being punished, it was only for four hundred years! It's clear from the OT that the Messiah would be coming sooner rather than later.
Arguments against the Old Testament from science are irrelevant. The Old Testament was never supposed to be a literal history of the Universe. It was an introduction to God for a people who did not yet know Him. He spoke in their language, according to what they could accept at that time. Later, in Jesus's time, he updated the teachings to include the rest of humanity.
Did evolution happen? Of course it did. Is the universe billions of years old and unimaginably vast? Of course. These things only prove God's existence all the more so! Evolution is far too intricate to lead to a being as astounding as the human without being guided somehow. A 4-billion year old Earth with a long, winding path towards the human reveals an incomprehensibly powerful God.
And what of Christ's teachings? Can anyone deny that his words carry immense wisdom?
Blessed are the peacemakers: for they shall be called the children
of God... Resist not evil: but whosoever shall smite thee on thy right cheek, turn to him the other also... I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despite-fully use you, and persecute you; --Matthew 5
If any one of you is without sin, let him be the first to cast a stone at her. --John 8:7
Do not judge, lest you too be judged. For in the same way you judge others, you will be judged and with the measure you use, it will be measured to you. --Matthew 7:1-2
Were not these teachings shown to be wise by Gandhi and Martin Luther King, Jr.? Who among modern figures are more revered than those two who so closely followed Jesus's teachings.
Oh, you will say, so much harm has been done in Jesus's name. This is true, I agree, but those so-called followers of Christ aren't doing any following. Christ predicted such hypocrisy and gave us the only test we need for discovering who the real Christians are. He said, "By their fruits you shall know them." In other words, if one praises Jesus but steals from the poor, he's not a Christian. If one praises Jesus but agitates for war, he's not a Christian. If one praises Jesus but bilks the faithful out of hundreds of thousands of dollars, he's not a Christian.
Mankind is flawed, and Jesus came to rescue us from ourselves. His sacrifice mirrors the primitive use of the (literal) scapegoat in the old testament, but reinvents it for a more enlightened humanity. Even to the gravest suffering, stand up for what's right, is his message. In the OT, the faithful kill the scapegoat. In the NT, the most faithful one IS the scapegoat. Don't strike, but turn the other cheek. When we all follow Jesus, there will be no war.
And what of our sins? What does it mean to say Jesus died to save us? By his death he teaches that living isn't the most important thing, how you live is. He modeled for us even as he was dying the perfect behavior. "Forgive them Father," he says, referring to his murderers, "they know not what they do." Christ died teaches us the alternative to hatred.